LONDON — A British warship forced three Iranian boats to back off after they sought to block a British tanker from passing through the Strait of Hormuz, the Defense Ministry said on Thursday, in the latest escalation of tensions between Tehran and the West.
“Contrary to international law, three Iranian vessels attempted to impede the passage of a commercial vessel, British Heritage, through the Strait of Hormuz,” the British government said. “We are concerned by this action and continue to urge the Iranian authorities to de-escalate the situation in the region.”
Iran denied any attempt to stop the tanker, according to Iranian news agencies.
The dispute on Thursday is the latest complication in a three-way drama involving Iran, the United States and Europe that has played out since last year, when President Trump abandoned the 2015 nuclear deal with Iran and restored sanctions that had been suspended under the deal.
Many analysts and diplomats have warned that each small collision like the encounter with the tanker on Thursday may now increase the risk of a larger conflagration.
Last week, British forces seized an Iranian tanker off the coast of Gibraltar bound for Syria, on suspicion that it was violating European Union sanctions, which Iran called an act of piracy. Some Iranian officials spoke of retribution, and an officer in Iran’s elite Islamic Revolutionary Guards Corps, Mohsen Rezaei, wrote on Twitter that if the tanker were not released, Iran “will be duty-bound to take reciprocal action and seize a British oil tanker.”
Why This Narrow Strait Next to Iran Is So Critical to the World’s Oil Supply
Twenty percent of the global oil supply flows past Iran through the Strait of Hormuz.
Iranian President Hassan Rouhani derided Britain as recently as Wednesday for its decision to send its warships to escort commercial vessels through the Gulf. Calling Britain “scared” and “hopeless,” Mr. Rouhani warned that, “You, Britain, are the initiator of insecurity and you will realize the consequences later.”
Britain and other European powers have sought to tamp down rising tensions between the United States and Iran, and salvage the nuclear pact. But Britain’s clashes with Iran could increase its own willingness to join the Trump administration in confronting Tehran, adding momentum to the conflict.
The most recent escalation began in May, after the Trump administration implemented comprehensive new sanctions intended to cut off all Iranian oil exports, as part of an effort to pressure Tehran into accepting sweeping new restrictions on its military activities and its nuclear program.
In May and June, six tankers were attacked near the Strait of Hormuz, and Washington said Iranian forces planted naval mines to target some of the ships. The British government has said publicly Iran was “almost certainly” responsible for an attack last month on two tankers in the Gulf of Oman. Iran has denied responsibility.
The United States was on the verge of launching a missile strike against Iran last month in retaliation for the shooting down of an American surveillance drone, but President Trump said he called off the military action at the last minute.
In the tanker seizure near Gibraltar, Britain has said the ship was stopped because of European Union sanctions against Syria, not because of American sanctions against Iran. But Spanish and Iranian officials have said the British acted at Washington’s request.
In its statement on Thursday, the British government said three Iranian boats had attempted to stop the British Heritage in the early morning as it headed toward the Strait of Hormuz, a narrow passage out of the Persian Gulf that is a vital channel for international oil supplies.
A British warship, the Montrose, had been escorting the tanker in an effort to guard against any Iranian interference. After a short standoff, the British warned the three Iranian boats to back away and they did, the British government said in its statement.
“H.M.S. Montrose was forced to position herself between the Iranian vessels and British Heritage and issue verbal warnings to the Iranian vessels, which then turned away,” the Defense Ministry said in a statement.
“Obviously very concerning developments,” Jeremy Hunt, Britain’s foreign secretary, told Sky News, “but also I’m very proud of the Royal Navy and the role they played in keeping British assets, British shipping safe. We are continuing to monitor the situation very, very carefully.”
Iran has long maintained that it does not seek a nuclear weapon. But the United States and some of its allies have been skeptical, citing evidence that Tehran was leaving that option open, and had hidden secret nuclear research that could move it closer to building a weapon.
Before the 2015 nuclear deal, the international powers had imposed economic sanctions intended to pressure Iran to accept restrictions on its nuclear program. Those sanctions were lifted when Iran agreed to dismantle much of its nuclear program under the deal.
The Trump administration later declared those restrictions inadequate and vowed to force Iran to submit to more severe limits on a much broader range of activities, including its support for allied militias around the region.
Britain and the European Union, which also signed the 2015 accord, still support the agreement and have urged Iran to comply with it, while pleading with the Trump administration to return to it. The European governments have also sought to set up an alternative trading system that would allow Iran to sidestep the latest American sanctions.
But the Trump administration has embarked on a campaign of “maximum pressure” that culminated in May with new sanctions seeking to cut off Iranian oil sales anywhere in the world, devastating the Iranian economy. Iranian officials called the penalties “economic warfare.”
After adhering to its obligations under the 2015 deal for more than a year after the United States withdrew, Tehran responded to the American actions with carefully calibrated steps to revive its nuclear program beyond the limits imposed by the accord.
Over the last two weeks, Iran has exceeded the cap on its stockpile of low-enriched uranium and also begun enriching uranium to slightly higher levels than had been allowed under the deal. Both moves could be easily reversed if Iran decides to return to full compliance with the agreement, but could be small steps toward building a nuclear weapon.
Iran argues that its steps are authorized under the agreement because European governments have failed to deliver the promised sanctions relief. It says it will take additional steps in sixty-day intervals unless the Western governments provide economic relief.
Under the terms of the accord, Britain or the other European nations could seek to trigger so-called “snapback sanctions,” restoring the former economic penalties against Iran to punish it for exceeding the limits of the agreement. That would effectively extinguish hopes of reviving the deal.
The United States and some of its allies have also accused Iran of retaliating against the sanctions by threatening the flow of oil through the vital Persian Gulf shipping lanes — a vulnerable passage with Iran on one side and its American-backed Arab rivals on the other.
The episode on Thursday will increase alarms in the West that Iran might seek to cut off the flow of oil through the Gulf.
The United States military has said it will begin working with partners to escort more tankers through the Gulf.
“It is hard to see an end game right now because it appears that the Trump administration is doubling down” but “the cost of that is going to be an increase in Iran’s escalatory reaction,” said Sanam Vakil, a researcher at Chatham House, in London. “It looks like we are going to be in simmering conflict for the near term.”