LONDON — A British warship tried without success to stop Iran’s seizure on Friday of an oil tanker near the Strait of Hormuz, an audio recording of the episode released on Sunday shows, underscoring the perils to Persian Gulf shipping and the global economy during escalating tensions between Iran and the West.
“If you obey, you will be safe,” an English-speaking officer from the Islamic Revolutionary Guards Corps of Iran says in a radio call to the tanker, Stena Impero, which was sailing under a British flag. “Alter your course to 360 degrees immediately, over.”
In another radio call, a Royal Navy officer aboard the British warship Montrose tells the tanker to stay its course.
“This is British warship foxtrot 236,” the officer says in the recording. “Sir, I reiterate that as you are conducting transit passage in a recognized international strait, under international law your passage must not be impaired, impeded, obstructed or hampered. Please confirm that you are conducting transit passage in a recognized international strait.”
The British said days earlier that the same warship, the Montrose, had inserted itself in front of a group of Iranian boats to successfully deter their attempt to capture another British-owned tanker. But on Friday, British officials said, the Montrose was too far from the Stena Impero to reach it in time.
In a separate video released by the semiofficial Iranian news agency Fars, small Iranian boats can be seen surrounding the Stena Impero. An Iranian helicopter flies above it, and several soldiers in black masks descend from the helicopter to take control of the tanker.
[Read: Britain warns Iran of “serious consequences” for seizing oil tanker.]
The audio of the capture, released by the maritime security firm Dryad Global, illustrates the difficult for even the comparatively formidable British Navy in protecting ships passing the Iranian coast through the Strait of Hormuz. A fifth of the world’s crude oil supply is carried through the narrow strait, a major choke point at the neck of the Persian Gulf.
The United States has talked of sharing intelligence and coordination to help other nations protect their commercial vessels passing through the gulf. But Washington has made clear that its role would be limited and that other nations must help shoulder the burden.
The recording may bolster British claims about the location of the ship at the moment of its capture, though it is possible that the naval officer was not speaking accurately about its location. The owner of the tanker, Stena Bulk, has said that it was in international waters at the time it was seized, and on Saturday, British officials said the ship was in Omani waters.
Iran has said that the tanker was sailing through Iranian waters, and has offered different explanations for its seizure, including that it had been polluting the gulf or had collided with an Iranian fishing boat.
Iranian officials, however, have also described the seizure as retribution for the British impounding of an Iranian tanker off the coast of Gibraltar on July 4. Britain has said that it stopped that tanker on suspicion that it had been violating a European Union embargo on the delivery of oil to Syria.
Iran has called that seizure an act of piracy directed by Washington as part of a pressure campaign against Tehran.
Britain’s foreign secretary, Jeremy Hunt, this weekend called the exchange a tanker “tit-for-tat.” But it is also part of a larger confrontation between Washington and Tehran over a 2015 agreement between world powers and Iran to limit the country’s nuclear program, which Western governments have feared might enable Iran to produce a nuclear weapon.
President Trump last year pulled the United States out of the agreement, vowing to force Iran to submit to more onerous and permanent restrictions on its conventional military and regional policies, as well as on its nuclear program.
The accord had promised Iran relief from economic sanctions in exchange for suspending and dismantling much of its nuclear program, but in May, Mr. Trump put in place sweeping new sanctions aimed at cutting off all exports of Iranian oil. The penalties have plunged the Iranian economy into crisis, and Iranian officials have described Mr. Trump’s moves as economic warfare.
Iran has responded with carefully calibrated steps to restart its nuclear program. But it has also flexed its military muscles, pointedly warning the world of its potential ability to strangle the passage of oil through the Strait of Hormuz.
The United States came within minutes of a missile strike in retaliation for the downing of the drone before Mr. Trump called the attack off, and last week, the United States said it had shot down an Iranian drone that had approached an American warship.
In addition to a reciprocal retaliation for the seizure of the Iranian tanker, the capture of the British tanker appeared to fit a larger Iranian strategy. Tehran has sought to raise the cost to the world powers of their effective failure to deliver on the promises of the 2015 nuclear accord because of the American sanctions. The rising exchange of threats has driven up oil prices and raised fears that a new military conflict may break out in the region.
Although Mr. Trump has signaled a willingness to negotiate a more limited resolution to the standoff over the renegotiation of the nuclear accord, Secretary of State Mike Pompeo on Saturday reiterated the administration’s demands that the Iranians “fundamentally change the direction of their nation.”
If the Iranian government decides “to behave like a normal nation,” Mr. Pompeo said during an appearance in Ecuador, “we’re prepared to negotiate across a broad spectrum of issues with no preconditions.”
But, he added, “We have seen no indications that the Iranians are prepared to fundamentally change the direction of their nation, to do the things we’ve asked them to do with their nuclear program, their missile program, their malign behavior around the world.”
Even before the tanker seizure, Britain occupied a pivotal position in the deadlock over the nuclear accord because it is the European country most hawkish toward Iran and most sympathetic to Washington.
Britain, France, Germany and the European Union all signed the 2015 agreement, and all have broken with the Trump administration to try to preserve it. Britain joined its European allies in attempting to construct an alternative trading platform that would allow the Iranians to bypass the American-linked global financial system and avoid the comprehensive new sanctions.
The tanker capture is, therefore, a risky gambit for Iran. It could push Britain closer to joining the United States in reimposing sanctions. That would virtually eliminate any hope of preserving the existing nuclear accord, even if in 2020 the United States elected a new president who sought to return to the deal.
For the moment, Britain is all but paralyzed by changes in its own leadership. Prime Minister Theresa May is expected to resign on Wednesday. A contest within the Conservative Party to succeed her is underway but has not yet concluded. And the favorite to win is the famously brash and unpredictable Boris Johnson.
Mr. Johnson, a former mayor of London and a former foreign minister, has said that he would seek to avoid a confrontation with Iran. But he has risen through his party largely by railing against Europe and has recently sought closer ties to Mr. Trump. Mr. Johnson was a leader of the campaign for Britain to exit the European Union, and as the Oct. 31 departure date approaches, he has suggested that improved trade with the United States could make up for any losses on the European side.
Mr. Hunt, the foreign minister, happens to be Mr. Johnson’s competitor in the runoff in the leadership contest. That has put a question mark over Mr. Hunt’s long-term future in the job, whether he ascends to be prime minister or is replaced under Mr. Johnson.
A week before the capture of the tanker on Friday, Mr. Hunt tried to reduce tensions with Tehran by suggesting that Britain could “facilitate” the release of the captured Iranian tanker in Gibraltar if evidence were to show that tanker had not been headed to Syria.
A court decision on Friday in Gibraltar, however, appears to have postponed that possibility. A judge ordered the continued detention of the ship until at least Aug. 15, pending another hearing.