Dubai, July 11 -- Britain said Thursday that three Iranian vessels unsuccessfully tried to impede the passage of a British commercial vessel through the Strait of Hormuz, signaling a further escalation of tensions over a 2015 nuclear deal between Iran and world powers.

The Iranian vessels only turned away after receiving "verbal warnings" from a U.K. navy vessel accompanying the commercial vessel British Heritage, the U.K. government 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," it said.

Iran's Revolutionary Guard denied the allegations, saying if it had received orders to seize any ships it would have executed them immediately.

The semi-o...