JERUSALEM — The sun was rising on the Mediterranean one recent morning when the crew of an Iranian cargo ship heard an explosion. The ship, the Shahr e Kord, was about 50 miles off the coast of Israel, and from the bridge the sailors saw a plume of smoke rising from one of the hundreds of containers stacked on deck.
The state-run Iranian shipping company said the vessel had been heading to Spain and called the explosion a “terrorist act.”
But the attack on the Shahr e Kord this month was just one of the latest salvos in a long-running covert conflict between Israel and Iran. An Israeli official said the attack was retaliation for an Iranian assault on an Israeli cargo ship last month.
Since 2019, Israel has been attacking ships carrying Iranian oil and weapons through the eastern Mediterranean and Red Seas, opening a new maritime front in a regional shadow war that had previously played out by land and in the air.
Iran appears to have quietly responded with its own clandestine attacks. The latest came on Thursday afternoon, when an Israeli-owned container ship, the Lori, was hit by an Iranian missile in the Arabian Sea, an Israeli official said. No casualties or significant damage were reported.
The Israeli campaign, confirmed by American, Israeli and Iranian officials, has become a linchpin of Israel’s effort to curb Iran’s military influence in the Middle East and stymie Iranian efforts to circumvent American sanctions on its oil industry.
But the conflict’s expansion risks the escalation of what has been a relatively limited tit-for-tat, and it further complicates efforts by the Biden administration to persuade Iran to reintroduce limits on its nuclear program in exchange for sanctions relief.
“This is a full-fledged cold war that risks turning hot with a single mistake,” said Ali Vaez, Iran program director at the International Crisis Group, a Brussels-based research organization. “We’re still in an escalatory spiral that risks getting out of control.”
Since 2019, Israeli commandos have attacked at least 10 ships carrying Iranian cargo, according to an American official and a former senior Israeli official. The real number of targeted ships may be higher than 20, according to an Iranian Oil Ministry official, an adviser to the ministry and an oil trader.
The Israeli attacks were first reported by The Wall Street Journal.
Most of the ships were carrying fuel from Iran to its ally Syria, and two carried military equipment, according to an American official and two senior Israeli officials. An American official and an Israeli official said the Shahr e Kord was carrying military equipment toward Syria.
The Israeli government declined to comment.
The extent of Iran’s retaliation is unclear. Most of the attacks are carried out clandestinely and with no public claims of responsibility.
The Israeli ship attacked last month was a car freighter, the Helios Ray, carrying several thousand German-made cars to China.
As the ship rounded the Strait of Hormuz, a narrow passage off the coast of Iran, a speedboat that had been trailing in its wake accelerated, zipping alongside the freighter. Commandos affixed two timed explosives to the port side of the ship, a meter above the water, according to a person with knowledge of the subsequent investigation.
Twenty minutes later the explosives ripped two holes in the hull.
Several tankers were similarly attacked in the Red Sea last fall and winter, actions some officials attributed to the Houthis, an Iran-backed rebel movement in Yemen.
Iran has denied involvement in all of these attacks which, like the Israeli ones, appeared intended not to sink the ships but to send a message.
“You attack us here, we’ll attack you there,” said Gheis Ghoreishi, a political analyst who has advised Iran’s Foreign Ministry on Middle Eastern affairs. “Iran and Israel are bringing their covert war to the open waters.”
The long-running shadow war between Israel and Iran has accelerated in recent years. Iran has been arming and financing militias throughout the region, notably in Syria, Iraq, Yemen, Gaza and Lebanon, where it supports Hezbollah, a Shiite militia and political movement that is a longtime enemy of Israel.
Israel has tried to counter Iran’s power play by launching regular airstrikes on Iranian shipments by land and air of arms and other cargo to Syria and Lebanon. Those attacks have made those routes riskier and shifted at least some of the weapons transit, and the conflict, to the sea, analysts said.
Israel has also sought to undermine Iran’s nuclear program through assassinations and sabotage on Iranian soil, and both sides are accused of cyberattacks, including a failed Iranian attack on an Israeli municipal water system last April and a retaliatory Israeli strike on a major Iranian port.
Iran’s Quds force was blamed for a bomb that exploded near Israel’s embassy in New Delhi in January. And 15 militants linked to Iran were arrested last month in Ethiopia for plotting to attack Israeli, American and Emirati targets.
The sum is an undeclared conflict that neither side wants to escalate into frontal combat.
“Neither Israel nor Iran want to publicly take responsibility for the attacks because doing so would be an act of war with military consequences,” Hossein Dalirian, a military analyst affiliated with Iran’s Islamic Revolutionary Guards Corps, told The New York Times in a Clubhouse discussion on Thursday. “But attacks against ships at this level could not happen without a state behind it.”
“We are at war but with our lights off,” he added.
The dynamic complicates already fraught efforts by the Biden administration to reconstruct the 2015 nuclear deal that imposed limits on Iran’s nuclear enrichment program in exchange for sanctions relief. President Donald J. Trump withdrew from the deal in 2018, reinstating those sanctions and imposing a raft of new ones.
“It jacks up the political price that the Biden administration would have to pay to provide the Iranians with any kind of economic reprieve,” Mr. Vaez said. “If Iran is engaged in this kind of tit for tat with Israel, while also putting pressure on American presence in the region, it makes restoring the deal much more difficult.”
Analysts say that Iran wants to continue to needle Israel and to arm and support its Middle Eastern allies, both to surround Israel with well-armed proxies and to give Iran a stronger hand in any future nuclear negotiations.
Israel’s leadership believes the previous nuclear deal was insufficient and would like to scuttle any chance of resurrecting a similar pact. An Israeli official said the attacks were part of a broader strategy to strong-arm Tehran into agreeing to tougher and longer curbs on its nuclear ambitions, as well as restrictions on its ballistic missile program and its support for regional militias.
That campaign, The Times previously reported, also included an Israeli attack on a major Iranian nuclear site in July and the assassination of Iran’s top nuclear scientist last November. Israel has not publicly acknowledged either operation.
The Israeli offensive against Iranian shipping has two goals, analysts and officials said. The first is to prevent Tehran from sending equipment to Lebanon to help Hezbollah build a precision missile program, which Israel considers a strategic threat.
The second is to dry up an important source of oil revenue for Tehran, building on the pressure American sanctions have inflicted. After the United States imposed sanctions on Iran’s fuel industry in late 2018, the Iranian government became more reliant on clandestine shipping.
The attacks were carried out by Flotilla 13, an elite commando unit of the Israeli Navy that has been involved in clandestine operations since the early years of the Israeli state, according to the two Israeli officials and the American official.
Israeli officials said that two of the ships it attacked were transporting equipment for Hezbollah’s missile program.
One, they said, was carrying an industrial planetary mixer, a device used to make solid rocket fuel for missiles. The device was meant to replace an older mixer that was destroyed in an Israeli airstrike on Beirut in August 2019, the Israeli officials said.
Previous Israeli airstrikes on Iranian convoys and cargo in Syria also targeted equipment for making guided missiles.
The tankers targeted by Israel were carrying Iranian oil to Syria, contravening American sanctions and most likely worth hundreds of millions of dollars.
The Israeli officials said that Syria paid Iran in cash or by providing logistical assistance to Syrian-based members of Iran’s Quds Force, a branch of the Revolutionary Guards, and to Hezbollah.
President Bashar al-Assad of Syria, also under sanctions, is in dire need of oil. Iran, its economy decimated by American sanctions, needs cash. Hezbollah has also been hit hard by the severe economic and political crisis in Lebanon and a cyberattack on its financial system.
The Israeli attacks are therefore “a way to prevent Iran from selling to Syria, and getting money and giving it to Hezbollah,” said Sima Shine, a former head of research at Mossad, the Israeli intelligence agency.
The attacks typically feature limpet mines and sometimes torpedoes, the American official said. They generally target the ships’ engines or propellers, one Israeli official said. And they are intended to cripple but not sink the ships, the American and Israeli officials said.
The attacks escalated toward the end of 2020, as Mr. Trump’s term drew to close. In response, Iran’s Revolutionary Guards began to discreetly escort the tankers through the Red Sea, before ships from Russia, an Iranian ally, accompanied them at a distance through the Mediterranean, the American official said.
The attack on the Shahr e Kord occurred when the Russian escort was far enough away for the Israelis to strike, the official added.
The effectiveness of the Israeli campaign is unclear. Some of the targeted ships were forced to return to Iran without delivering their cargo, the American official said.
The Iranians associated with the Iranian Oil Ministry said that in all cases the vessels sustained minor damage, the crews were not hurt and repairs were conducted within a few days.
The American and Israeli officials said there was no connection between the Israeli campaign and a recent oil spill that left tons of tar on the beaches of Israel and Lebanon.
Within Israel, there is concern among maritime experts that the cost of a sea war may exceed its benefit.
While the Israeli Navy can make its presence felt in the Mediterranean and Red Seas, it is less effective in waters closer to Iran. And that could make Israeli-owned ships more vulnerable to Iranian attacks as they pass Iran’s western shores on their way to ports in the Gulf, said Shaul Chorev, a retired Israeli admiral who now heads the Maritime Policy and Strategy Research Center at the University of Haifa.
“Israeli strategic interests in the Persian Gulf and related waterways will undoubtedly grow,” he wrote in a statement, “and the Israeli Navy does not have the capabilities to protect these interests.”
Patrick Kingsley reported from Jerusalem, Ronen Bergman from Tel Aviv, Farnaz Fassihi from New York, and Eric Schmitt from Washington.