LONDON — Britain on Saturday warned ships to stay out of the Strait of Hormuz, a crucial shipping route for the world’s oil supplies, as the government pressed Iran to release a British-owned oil tanker seized the previous evening, dramatically escalating tensions in the region.
“We have advised U.K. shipping to stay out of the area for an interim period,” the British government said in a statement released early Saturday after an emergency meeting. “There will be serious consequences if the situation is not resolved,” the government warned.
The capture of the tanker is a sharp step up after three months of rising tensions between Iran and the West that last month brought the United States within minutes of a military strike against targets in Iran.
A fifth of the world’s crude oil supply is shipped from the Persian Gulf through the narrow Strait of Hormuz off the coast of Iran, and oil prices spiked sharply on Friday even before the British warning.
Iranian news agencies reported that all 23 crew members of the British-flagged tanker would be held onboard in the Bandar Abbas Port in Iran during a criminal investigation of the ship’s actions. The nationalities of those crew members included Indian, Russian, Latvian and Filipino, the ship’s owner said in a statement.
The Iranian authorities also added reasons for the seizure of the ship, saying for the first time on Saturday that the vessel had been involved in an accident with an Iranian fishing boat and that the tanker had ignored distress calls.
The Islamic Revolutionary Guards Corps, which is in charge of Iranian naval activities in the Persian Gulf, said on Friday that it had seized the ship for deviating from traffic patterns and polluting the waters, but it had not mentioned any episode with a fishing boat.
Stena Bulk, the owner of the ship, Stena Impero, said the tanker had been in “full compliance with all navigation and international regulations” when it was intercepted.
In Washington on Friday, President Trump called Iran “nothing but trouble.”
“We’ll be working with the U.K.,” Mr. Trump told reporters on the South Lawn of the White House. Referring in vague terms to the close American alliance with Britain, he added, “We have no written agreement, but I think we have an agreement that is longstanding.”
United States Central Command, the division of the military that oversees the Middle East, repeated in a statement late Friday that it was working on a “multinational effort” under the name Operation Sentinel to police the shipping routes.
The operation “will enable nations to provide escort to their flagged vessels while taking advantage of the cooperation of participating nations for coordination and enhanced maritime domain awareness and surveillance,” the statement said.
But it emphasized that Washington would not shoulder the burden alone: “While the United States has committed to supporting this initiative, contributions and leadership from regional and international partners will be required to succeed.”
France on Saturday called on Iran to respect “the principle of freedom of shipping in the Gulf,” according to a statement on the website of the Ministry of Foreign Affairs. The German government strongly condemned Iran’s actions in a statement on Saturday, calling the seizure “unjustifiable.” “Another regional escalation would be very dangerous,” the statement said, adding it would “undermine all ongoing efforts to find a way out of the current crisis.”
Both countries urged Iran to free the ship and its crew as soon as possible.
The back-and-forth between Iran and the West has already included the imposition of sweeping new American economic sanctions followed by the calibrated resumption of an Iranian nuclear energy program that the West fears might lead to a nuclear bomb. The United States and Britain have accused Iran of sabotaging six tankers in a tacit threat to gulf shipping routes. Both United States and Iran have said it had shot down an unpiloted surveillance drone flown by the other side.
In a vivid reminder that each minor collision risks the explosion of a more violent confrontation, Mr. Trump initially ordered the missile strike last month in retaliation for the Iranian downing of an American surveillance drone. He called the strike off only minutes before the launch.
Mr. Trump said the next day that he had concluded the missile strike would have been a disproportionate response. Then he later threatened the “obliteration” of parts of Iran if it targeted “anything American.”
At the core of the confrontation with the West is the Trump administration’s attempt to rip up and renegotiate a 2015 accord that the United States and other world powers had reached with Iran to limit its nuclear program in exchange for relief from economic sanctions.
Having pulled the United States out of the deal last year, the Trump administration added comprehensive sanctions in May that were intended to block all of Iran’s oil exports, the lifeblood of its economy. Iranian officials denounced the new penalties as “economic warfare.”
Iran has sought to push back against all the major powers, forcing them to feel some cost for their effective default on the promises 2015 accord as a result of Mr. Trump’s sanctions. The conflict with Washington has set the backdrop for a parallel clash with Britain that led to the seizure of the tanker.
This is a delicate moment for Britain. Its governing Conservative Party this coming week will pick a new prime minister — either Boris Johnson or Jeremy Hunt — and is set to leave the European Union on Oct. 31.
The current confrontation began this month, when the British military helped impound an Iranian tanker off the coast of Gibraltar on the suspicion that it was on its way to deliver oil to Syria in violation of European Union embargoes.
Iranian officials called the seizure of their ship an act of piracy and accused Washington of masterminding the capture as part of its pressure campaign. Officers of the Revolutionary Guards threatened to retaliate against a British ship, and Iran appears to have done so on Friday.
Mr. Hunt, the British foreign secretary, on Saturday charged Iran with violating international law but asserted that Britain had followed proper legal procedures in stopping the Iranian tanker, Grace 1, near Gibraltar.
“Yesterday’s action in Gulf shows worrying signs Iran may be choosing a dangerous path of illegal and destabilising behaviour after Gibraltar’s LEGAL detention of oil bound for Syria,” Mr. Hunt wrote on Twitter Saturday morning.
“As I said yesterday our reaction will be considered but robust,” Mr. Hunt added. “We have been trying to find a way to resolve Grace1 issue but WILL ensure the safety of our shipping.”
Britain has so far joined the other world powers in seeking to preserve the nuclear deal with Iran despite Mr. Trump’s opposition. European states have even attempted to set up an alternative trading platform that would allow Iran to evade the American sanctions.
But of all the European powers, Britain is the most dubious toward Iran. If Britain chooses to join the United States in re-imposing sanctions on Iran, that would all but completely snuff out any hope of saving the 2015 accord.