LONDON — Iran seized at least one British oil tanker in a vital Persian Gulf waterway on Friday, a sharp escalation of tensions with the West that revived fears of a military clash, even as voices on both sides appeared to be seeking room for negotiations.
The impoundment of the tanker by Islamic Revolutionary Guards Corps naval patrols came a day after the United States said it had downed an Iranian drone menacing an American warship in the region.
But Iran’s standoff with Britain, in particular, carries its own complications. Britain occupies a pivotal place in a bloc of European states that have tried to broker some resolution to a broader conflict between Tehran and Washington over the fate of a 2015 deal with the world powers designed to prevent Iran from obtaining a nuclear weapon.
Prime Minister Theresa May of Britain convened an emergency meeting of advisers late Friday night to respond.
Jeremy Hunt, the foreign secretary, said in a statement issued before the meeting that he was “extremely concerned” and called the seizure “unacceptable.”
At the time Mr. Hunt spoke, Iran had at least briefly detained a second British-owned ship, and Mr. Hunt said the meeting would address “what we can do to swiftly secure the release of the two vessels.” He noted that no British citizen had been among the crews.
“We’re not looking at military options; we’re looking at a diplomatic way to resolve the situation,” Mr. Hunt said later. “But we are very clear that it must be resolved.”
The United States Central Command, which is responsible for U.S. operations in the Middle East, said in a statement that “patrol aircraft in international airspace” were monitoring the Strait of Hormuz and the Navy was in contact with American ships in the area “to ensure their safety.”
The display of force by the Revolutionary Guards was publicly welcomed by hard-line Iranian officials. Iran’s supreme leader, Ayatollah Ali Khamenei, whose hostility toward Britain and the United States is well known, appeared to revel in the achievement of capturing the British vessel.
“The country’s proud defense capabilities are a result of the pressures and cutting ties with foreigners” during the era of Iran’s long war with Iraq in the 1980s, the ayatollah said in a post on social media.
He also appeared to encourage Iranians to persevere through the crippling economic sanctions that were imposed by the United States in May and set off the current escalation.
“The movement now to rely on only ourselves will yield important results including economically,” the ayatollah said.
Tensions between Britain and Iran spiked earlier this month when the British military impounded an Iranian tanker near Gibraltar on suspicion of having violated a European Union embargo on the sale of oil to Syria. Iran called the seizure “piracy,” accused Britain of acting on a pretext at the behest of Washington and threatened to capture a British ship in retaliation.
Iranian vessels first tried to stop a British tanker in the Persian Gulf region a few days later, on July 11. After a short standoff, an accompanying British warship drove them away.
But late Friday afternoon, Iranian news agencies reported that Revolutionary Guard seamen had indeed seized at least one British tanker, the Stena Impero, in the Strait of Hormuz, the narrow Gulf waterway that is a critical conduit for maritime oil traffic.
The news agencies quoted the Guards as saying the tanker had “violated three international naval regulations,” including turning off a GPS locator, breaking the traffic pattern in the Strait of Hormuz and polluting the water by dumping crude oil residue.
“We asked the armed forces to guide this tanker to Bandar Abbas port so we can investigate further,” Allah Morad Anifipour, the head of Iran’s shipping and port organization, said, according to official Iranian accounts.
The ship’s owners reported that the Stena Impero, a 30,000-ton vessel bound for Saudi Arabia, had been “approached by unidentified small crafts and a helicopter during transit of the Strait of Hormuz while the vessel was in international waters.”
“We are presently unable to contact the vessel which is now heading north toward Iran,” the owners, Stena Bulk, and the ship’s managers, Northern Marine, said in the statement.
The second tanker, at least temporarily detained, was the Mesdar, owned and operated by the Glasgow-based shipping firm Norbulk, but flying a Liberian flag. It too lost contact for a time, but Fars, the semiofficial Iranian news agency, reported that the Iranian authorities had only warned it to abide by environmental regulations.
It was unclear late Friday if the British authorities had confirmed the release of the second tanker.
At least one senior American military official on Friday appeared to play down the latest escalation by Iran, calling it a foreseeable response to the British seizure of the Iranian tanker near Gibraltar.
“They look for things that are proportional in nature,” Lt. Gen Robert P. Ashley Jr., the top military intelligence officer, said in a discussion with journalists at the Aspen Security Forum in Colorado. “They aren’t looking to go to war but at the same time they are looking to project strength,” he said.
The capture of the Stena Impero followed an increasingly heated exchange of threats between Iran and Washington, set off by the Trump administration’s attempts to scrap and renegotiate the 2015 nuclear accord, under which the United States and six other world powers, including Britain, promised Iran relief from economic sanctions in exchange for limits on its nuclear program.
After pulling out last year, President Trump in May imposed new sanctions that seek to block all exports of Iranian oil, the mainstay of its economy.
Denouncing the sanctions as “economic warfare,” Iran has sought to put pressure on Washington and its European allies by taking gradual steps to exceed its own commitments under the deal to dismantle and suspend its nuclear program.
Britain has so far continued to try to preserve the 2015 deal in defiance of the Trump administration. Along with the other European powers, Britain has largely accepted Iran’s position that its steps to restart its nuclear program are justified under the terms of the deal as responses to the reimposition of American sanctions.
Britain has even joined other Europeans in attempting to develop an alternative trading system that would allow Iran to bypass the U.S. sanctions.
But among the European powers, diplomats say, Britain is also the most skeptical of Iran and the most sympathetic to the White House. If Britain now chooses to re-impose its own sanctions on Iran, that would all but completely extinguish any hope of preserving the nuclear deal.
At the same time, the United States and its allies have accused Tehran of using naval mines to damage six ships in two attacks in the Persian Gulf, evidently in a tacit threat to the crucial oil shipping lanes that flow past Iran through the Strait of Hormuz. Iran has denied carrying out those attacks but it boasted that it shot down an American surveillance drone last month.
Trump, in response, ordered a missile strike on Iran, only to call it off only minutes before launch.
On Thursday, a day before the capture of the British tanker, the United States said that it had brought down an Iranian drone that had come too close to an American amphibious assault ship. “The latest of many provocative and hostile actions by Iran,” Mr. Trump called it.
Iranian officials, however, refused to acknowledge that any of their drones were shot down. A “false claim rooted in Trump’s illusions,” Gen. Abolfazi Shekarchi was quoted as saying by Iranian news agencies.
While some voices on each side have continued to ratchet up the bluster, others have in recent days seemed to probe for paths out of the confrontation. Mr. Trump, sometimes sounding at odds with his more hawkish advisers, has repeatedly said he would be open to negotiations without preconditions. This week, Republican Senator Rand Paul of Kentucky offered himself as a mediator.
Iranian officials have offered their own olive branches. Speaking with American journalists in New York while attending a United Nations meeting, Foreign Minister Mohammad Javad Zarif appeared to seek to break the impasse by suggesting that Iran might submit to more comprehensive international inspections of its research facilities in exchange for a revival of the deal for sanctions relief.
The seizure of the British tanker on Friday, however, threatened to sideline such diplomacy, and other Iranian hard-liners celebrated it as a triumph.
“The Persian Gulf will always belong to us,” Morteza Avini, a conservative filmmaker who documents Iran’s conflicts, wrote on social media. The capture, he wrote, “means that even the United States and Britain must abide by the rules that we set. This means a powerful Iran.”