LONDON -- Iran seized a British-flagged oil tanker Friday and briefly detained a second vessel in the Strait of Hormuz, intensifying tensions in the strategic waterway that has become a flashpoint between Tehran and the West.

The seizing of the Stena Impero marked perhaps the most significant escalation of the friction since May, when the United States dispatched an aircraft carrier to the Persian Gulf amid claims of unspecified threats by Iran.

The ongoing showdown has caused jitters around the globe, with each manoeuvr bringing fear that any misunderstanding or misstep by either side could lead to war.

Details of what took place Friday remained sketchy after Iran reported that it had seized a British oil tanker in the Strait of Hormuz. The straight at the mouth of the Persian Gulf is a shipping channel for one-fifth of all global crude exports.

The Stena Impero was taken to an Iranian port because it was not complying with "international maritime laws and regulations," Iran's Revolutionary Guard declared.

A statement from Stena Bulk, which owns the seized tanker, said it was unable to make contact with the ship after it was approached by unidentified vessels and a helicopter in international waters.

The company said the tanker had 23 crew members of various nationalities and there were no reports of any of them were injured.

The U.K. has featured prominently in the recent tensions with Iran. Britain's Royal Marines assisted in the seizure of an Iranian oil supertanker on July 4 by Gibraltar, a British overseas territory off the southern coast of Spain.

Britain said it would release the vessel if Iran could prove it was not breaching European Union sanctions on oil shipments to Syria.

Gibraltar's government said Friday that its Supreme Court had extended by 30 days the detention of the Panama-flagged Grace, which was loaded with over 2 million barrels of Iranian crude oil.

British Foreign Secretary Jeremy Hunt initially said two ships were seized Friday in the Strait of Hormuz, the second sailing under a Liberian flag.

The owner of the Liberian-flagged tanker later said the ship was briefly boarded by armed guards before being allowed to go. Iran's semi-official Fars news agency tweeted that the Mesdar had left Iran's territorial waters.

"These seizures are unacceptable," Hunt said as he prepared to enter an emergency government meeting Friday night. "It is essential that freedom of navigation is maintained and that all ships can move safely and freely in the region."

U.K. Chamber of Shipping chief executive Bob Sanguinetti said the seizure represented a severe escalation of tensions in the Gulf and made it clear that merchant vessels urgently needed more protection.

The British government should do "whatever is necessary" to ensure the safe and swift return of the ship's crew, Sanguinetti said.

President Donald Trump said U.S. officials would talk with Britain about the unfolding crisis.

"This only goes to show what I'm saying about Iran: Trouble, nothing but trouble," he said.

The incident came two days after Washington claimed that a U.S. warship downed an Iranian drone in the strait. Iran denied that it lost an aircraft in the area.

On June 20, Iran shot down an American drone in the same waterway, and Trump came close to retaliating but called off an airstrike at the last moment.

Tensions in the region have been escalating since Trump withdrew the U.S. last year from Iran's 2015 nuclear deal with world powers and imposed sweeping economic sanctions on Iran, including its oil exports. The sanctions have hit the Iranian economy hard.

Iran's government has desperately tried to get out of the chokehold, pressuring the other partners in the nuclear deal, particularly European nations, to pressure the U.S. to lift the crippling sanctions.

The Europeans -- Germany, France, Britain, and the European Union -- want to maintain the deal, but have not been able to address Iranian demands without violating the sanctions. Iran has begun breaching some of the restrictions on its activities outlined in the agreement to put pressure on them to find a solution.

The U.S. has asked Mideast allies like Saudi Arabia and the United Arab Emirates in past weeks to contribute financially and militarily to a Trump administration proposal called the Sentinel Program -- a coalition of nations working with the U.S. to preserve maritime security in the Persian Gulf and keep eyes on Iran.

Before the British ship was seized, Iran and the United States disagreed over Washington's claim that a U.S. warship downed the Iranian drone. American officials said they used electronic jamming to bring down the unmanned aircraft, while Iran said it simply didn't happen.

Neither side provided evidence to prove its claim.

At the White House, Trump said flatly of the Iranian drone: "We shot it down." But Pentagon and other officials have said repeatedly that the USS Boxer, a Navy ship in the Strait of Hormuz, actually jammed the drone's signal, causing it to crash, and did not fire a missile. The officials spoke on condition of anonymity to discuss sensitive technology.

In Tehran, the Iranian military said all its drones returned safely to their bases and denied there was a confrontation with the USS Boxer, an amphibious assault ship.

"We have not lost any drone in the Strait of Hormuz nor anywhere else," tweeted Deputy Foreign Minister Abbas Araghchi.

The Iranians and Americans have had close encounters in the Strait of Hormuz in the past, and it is not unprecedented for Iran to fly a drone near a U.S. warship.

The Revolutionary Guard said its forces continue to monitor all movements by foreigners -- especially "the terrorist forces" of the U.S. and the British in the Strait and the Gulf. It had earlier threatened to seize a British tanker.

U.K.-flagged vessels represented less than 0.6% of the 67,533 ships sailing through the Strait of Hormuz in 2018, with 427 transits, according to maritime publication Lloyd's List, quoting research from Russel Group.

Crude oil prices climbed following Iran's announcement about the Stena Impero as traders worried the escalating tensions could affect crude supplies.

The price of oil was lower earlier in the day. U.S. stocks also fell as traders sold to avoid holding stocks into the weekend.