The seizure of the British tanker by Iran has marked perhaps the most significant escalation since tensions between Iran and the West began rising in May.
Iran has seized a British-flagged oil tanker 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 British tanker marked perhaps the most significant escalation since tensions between Iran and the West began rising in May. At that time, the US announced it was dispatching an aircraft carrier and additional troops to the Middle East, citing unspecified threats posed by Iran.
The ongoing showdown has caused jitters around the globe, with each manoeuvre 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 UK 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."
UK 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 US 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.
Central Command said the US has intensified air patrols over the Strait of Hormuz in response to the seizure.
A Central Command spokesman, Lt. Col. Earl Brown, said a small number of additional patrol aircraft are flying in international airspace to monitor the situation.
The incident came two days after Washington claimed that a US 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 US 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 US 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.