DUBAI, United Arab Emirates — Iranian vessels in the Persian Gulf attempted to block a British tanker from traveling through the Strait of Hormuz, a key waterway for global oil shipments, Britain said Thursday, in what appeared to be a major escalation in tensions between Iran and Western powers.

The tanker British Heritage was approaching the northern entrance to the strait when its British naval escort, the HMS Montrose, “was forced to position herself between the Iranian vessels” and the ship, a government statement said.

The navy ship issued “verbal warnings to the Iranian vessels, which then turned away,” it continued. “We are concerned by this action and continue to urge the Iranian authorities to de-escalate the situation in the region,” the statement said.

Iran denied that its forces challenged the Isle of Man-flagged tanker, which is operated by the London-based oil and gas company BP. In a statement carried by Iranian news agencies, Iran’s Islamic Revolutionary Guard Corps said there had been no confrontations with foreign vessels in the past 24 hours.

A U.S. official, speaking on the condition of anonymity because he was not authorized to speak publicly, confirmed that an American aircraft captured footage of the Iranian ships interacting with the British vessels.

The incident comes amid soaring tensions between Iran and the West, including over Iran’s moves to expand uranium enrichment in a breach of its 2015 nuclear deal with world powers. Tehran says it is responding to U.S. pressure following the Trump administration’s decision to abandon the pact, which curbed Iran’s nuclear energy activities in exchange for major sanctions relief.

The administration reimposed a near-total embargo on Iran’s economy in the fall. Since then, Iran has urged the other signatories – including European nations – to deliver the economic benefits promised to Tehran under the agreement.

Last week, British forces assisted local law enforcement in apprehending an Iranian supertanker in the Mediterranean Sea near the British territory of Gibraltar – a move Iran denounced as “an act of piracy.” The vessel was carrying oil to the Syrian refinery at Banias in violation of European Union sanctions, officials in Gibraltar said.

Iranian Foreign Minister Mohammad Javad Zarif said Britain was trying to “increase tensions” by saying that the British Heritage was unable to pass through the Strait of Hormuz, calling the government’s claims “worthless.” He said the seizure of the Iranian vessel was part of a campaign of U.S.-led “economic terrorism” against Iran.

Vice Adm. Jim Malloy, who commands U.S. naval forces in the Middle East, said in a statement that the U.S. military is “aware of the Islamic Revolutionary Guard Corps Navy’s unlawful harassment and attempts to interfere with the passage” of the British Heritage.

“There is a global standard for professional maritime conduct, and together with our partners we remain committed to exemplifying these maritime customs, standards and norms, as professional mariners throughout the regional waterways, wherever international law allows,” Malloy said. “We continue to work closely with the Royal Navy, along with all our regional and global partners who share in our commitment to preserving and defending the free flow of commerce and the freedom of navigation.”

The British Heritage traversed the Strait of Hormuz overnight and was sailing through the Gulf of Oman on Thursday, according to open source tracking data. It was not believed to be carrying any cargo, according to shipping analysts.

The tanker had failed to load its 140,000-ton cargo of crude oil in Basra, Iraq, on July 4, the maritime intelligence site Lloyd’s List reported, and instead diverted to Saudi territorial waters in the gulf.

The site said that the British Heritage was chartered by Royal Dutch Shell, a British-Dutch oil and gas company headquartered in the Netherlands.

A BP spokesman declined to comment on the incident but said that the company’s “top priority is the safety and security of our crews and vessels,” the BBC reported.

“We thank the Royal Navy for their support,” the spokesman said.

The simmering confrontation between Iran and Britain has centered on the Iranian supertanker detained in Gibraltar July 4. Iran has supported the government of Syrian President Bashar Assad with troops, oil and assets throughout Syria’s yearslong civil war. Under a regulation adopted July 3, authorities are allowed to detain “specified ships” suspected of violating EU sanctions.

Authorities in Gibraltar arrested the captain and chief officer of Grace 1 on suspicion of breaching the sanctions Thursday, a spokesman for the Royal Gibraltar police said in a statement. Documents and electronic devices were also seized, the statement said.

“The enemy is going to regret this act,” Rear Adm. Ali Fadavi, a deputy Revolutionary Guard commander, said Thursday of the tanker’s seizure, Iran’s Tasnim News Agency reported. “They would not have done it if they had done the minimum calculation.”

Fadavi added: “We had rented this ship, and we carried the cargo. Their action was very silly, and they will certainly regret it.” He also blamed the U.S. government for the seizure.

“Our reciprocal action will be announced,” he said.

British Foreign Secretary Jeremy Hunt, a contender to succeed Theresa May as Conservative Party leader and prime minister, said that the developments were “very concerning” but that he was “also very proud of the Royal Navy and the role they played in keeping British assets, British shipping, safe.”

“We are continuing to monitor the situation very, very carefully,” he told the BBC.

A spokesman for May told reporters that Britain has a “long-standing maritime presence in the Persian Gulf. We are continuously monitoring the security situation there and are committed to maintaining freedom of navigation in accordance with international law.”

British naval forces maintain a permanent presence in the gulf, including helicopters, dock landing shops and an anti-mine squadron, according to the Defense Ministry’s website.

It says that there is typically at least one escort on notice for maritime security patrol in the area.

Richard Dalton, a former British ambassador to Iran, said that the denials from Iranian officials suggest the incident “wasn’t a calculated or authorized attempt” but rather a “political demonstration.”

He said that British officials “have been bracing themselves for retaliation” for the Grace 1 seizure.

The Iranians “tend to want to retaliate in situations like that and give the British a good reason for speeding up the release of this extremely valuable cargo,” Dalton said.