The tanker, British Heritage, was under escort by a warship, the Montrose, and after a brief standoff but no exchange of fire, the three Iranian boats complied with “verbal warnings” to retreat, the ministry said in a statement.

“We are concerned by this action and continue to urge the Iranian authorities to de-escalate the situation in the region,” the British government said. Iran denied any attempt to stop the tanker, according to official news agencies.

Big picture: The dispute is the latest complication in a three-way drama involving Iran, the U.S. and Europe that has played out since last year, when President Trump abandoned the 2015 nuclear deal with Iran and restored sanctions.

Among the European powers, Britain has been the most wary of Iran, diplomats say, making it a pivotal player in the deliberations over the future of the deal.

Reminder: Last week, British forces seized an Iranian tanker off the coast of Gibraltar bound for Syria, on suspicion that it was violating E.U. sanctions. Some Iranian officials spoke of retribution.