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-official Fars news agency carried a statement from the Guard’s navy early Thursday saying “there were no clashes with alien boats, especially English boats.”

The incident was reported a day after Iran’s president, Hassan Rouhani, warned that Britain will face “repercussions” over the seizure of an Iranian supertanker last week that authorities in Gibraltar suspect was breaching European sanctions on oil shipments to Syria.

Rouhani was quoted by Iranian state media as saying the seizure was “mean and wrong.”

Iranian Foreign Minister Mohammad Javad Zarif denied the supertanker belonged to Iran, saying whoever owned the oil shipment and the vessel could pursue the case through legal avenues.

Iran had earlier summoned the British ambassador over what it called the “illegal interception” of the ship.

It marked the latest escalation of tensions in the Persian Gulf over the nuclear deal.

Since Trump withdrew from the nuclear accord, the restoration of heavy sanctions on Iran, including its oil industry, has exacerbated an economic crisis that has sent the currency plummeting.

The U.S. has meanwhile sent thousands of troops, an aircraft carrier, nuclear-capable B-52 bombers and advanced fighter jets to the Middle East. Fears are growing of a wider conflict after mysterious oil tanker attacks near the Strait of Hormuz blamed on Iran, attacks by Iranian-backed rebels in Yemen on Saudi Arabia, and Iran’s downing of a U.S. military drone.

So far, the remaining parties to the nuclear deal — Russia, China, Germany, France, Britain and the European Union — have been unable to meet Tehran’s demands for enough economic assistance to offset the American sanctions.

In recent weeks, Iran began breaching the limits of the deal, both on the permitted stockpile of low-enriched uranium and the permitted level of uranium enrichment.

It also set an early September deadline for world powers to save the agreement, saying it would otherwise take a third step in going beyond the deal’s limits.

Iran maintains it is justified in breaching the limitations because the U.S. already broke the deal with its unilateral withdrawal last year.