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.