Britain offered on Saturday to return a seized Iranian tanker if Tehran provided guarantees that the oil would not go to Syria.
The offer appeared to be an effort to cool down relations between the two countries at a time when tensions between Iran and Washington have raised fears of a military conflict and threatened to unravel a 2015 deal constraining Iran’s nuclear program.
A parallel dispute between Britain and Iran broke out a little more than a week ago when the British military seized an Iranian oil tanker off the coast of Gibraltar. Britain said it suspected the tanker was heading to Syria, in violation of European Union sanctions.
Iran called the seizure an act of piracy, accused Britain of acting at the behest of Washington and threatened to retaliate in kind.
Then on Thursday, Britain said that three Iranian vessels had tried to stop a British oil tanker passing through the Strait of Hormuz. A British warship escorting the tankers had threatened the Iranian ships and driven them away without exchanging fire, the British Defense Ministry said.
In apparent attempt to forestall a further escalation, Jeremy Hunt, the British foreign minister, said on Twitter on Saturday that he had held a “constructive call” with his Iranian counterpart.
“I reassured him our concern was destination, not origin, of the oil” on the seized ship, Mr. Hunt said. He added that he had also said Britain would “facilitate release” of the impounded ship “if we received guarantees that it would not be going to Syria, following due process” in the Gibraltar courts.
A court in Gibraltar on Friday authorized the detention of the ship for an additional 14 days, citing evidence that it may have been headed for Syria. Iran is one of the most important international backers of the regime of Bashar al-Assad, the president of Syria.
Mr. Hunt said in a second tweet that Iranian Foreign Minister Mohammed Javad Zarif had responded “that Iran wants to resolve issue and is not seeking to escalate.”
It was unclear when or exactly how Iran might provide guarantees of the ship’s destination to the Gibraltar courts. But Britain’s desire to soothe the tensions is significant because of its potentially pivotal role in the larger conflict between Iran and Washington.
Britain was one of several European powers to join the United States in signing the 2015 accord that required Iran to suspend and dismantle most of its nuclear program in exchange for relief from economic sanctions. Iran has insisted that its nuclear program was intended only for peaceful purposes, but the United States and its allies have suspected that Iran was keeping open the possibility of building a nuclear weapon.
Breaking with the Trump administration, Britain and other European powers have now sought to save the deal, even as first the United States and now Iran have stopped complying with the terms. President Trump withdrew from the deal last year and has implemented new and increasingly painful economic sanctions against Iran, demanding that it submit to more restrictions on its military activities as well as its nuclear program.
Iran, in response, has recently taken carefully calibrated steps to exceed the limits on its enrichment and stockpiling of uranium.
Britain is the European power most skeptical of the Iranian stance.
The Iran’s tensions with Britain had increased the possibility that it might join the Trump administration in restoring its own sanctions against Iran, a move that could effectively kill any hope of reviving the nuclear deal.