Tensions are once again rising in the Strait of Hormuz, after the U.K.'s Ministry of Defense said three Iranian ships tried to block the passage of a British oil tanker and a British naval vessel had to intervene.

Iranian officials, however, denied that any such incident occurred.

The U.K. ministry said in a statement that on Wednesday, "HMS Montrose was forced to position herself between the Iranian vessels and British Heritage and issue verbal warnings to the Iranian vessels, which then turned away."

Montrose was shadowing the tanker in the waterway between Oman and Iran, when it "appeared that the Iranian vessels were trying to divert the Heritage from international to Iranian waters," a ministry source told CNN.

In Tehran, Foreign Minister Mohammad Javad Zarif told reporters Thursday that the U.K.'s account is "worthless and aimed at escalating tension," Iran's semi-official Fars news agency reported.

And Iran's Islamic Revolutionary Guard Corps said in a statement in Fars: "Naval patrols of the Guards Corps in the Persian Gulf continue vigilantly, precisely and strongly based on regular procedures and missions, and during the last 24 hours, there have been no encounters with foreign vessels, including the British ones."

The area around the Strait of Hormuz, a critically important shipping channel between the Persian Gulf and the Gulf of Oman, has been the site of several high-profile incidents in recent weeks.

The U.S. says Iran attacked the tankers Front Altair and Kokuka Courageous in the Gulf of Oman last month using magnetic mines. Iran has denied involvement.

A week later, Iran shot down a U.S. drone with a surface-to-air missile over the Strait of Hormuz. Tehran said the spy craft had violated its airspace, while the U.S. maintained it was in international waters close to Iran. The incident brought the U.S. to the brink of a military strike on Iran, before President Trump said he pulled back.

The Strait of Hormuz links oil producers in the Middle East with outside markets. Last year, an average of 21 million barrels of oil a day passed through the narrow stretch of water — the equivalent of about one-fifth of the world's oil consumption, according to the U.S. Energy Information Administration.

The commerical tanker said to have been protected by the Royal Navy on Wednesday was operated by British Petroleum.

"Our top priority is the safety and security of our crews and vessels," BP spokesman Paul Erwood told NPR in a statement. "While we are not commenting on these events, we thank the Royal Navy for their support."

Reports of an attempted interception comes a week after British Royal Marines helped seize an Iranian tanker in the Strait of Gibraltar that was suspected of carrying oil from Iran to Syria, in violation of EU sanctions.

Iran condemned the seizure as "maritime piracy," according to Fars, and "has vowed to employ all its political and legal capacities to secure the release of the vessel and uphold its rights."

As NPR's Peter Kenyon reported, "a former Revolutionary Guards commander was quoted as saying Iran should seize a British tanker in retaliation for the recent seizure of an Iranian tanker by the British Royal Marines."

Meanwhile, the Trump administration has threatened to "substantially" increase sanctions on Iran after Tehran announced that it had breached several limits under the 2015 nuclear deal. The U.S. pulled out of that deal last year.

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