US Secretary of State Mike Pompeo has accused Iran of being behind attacks on two oil tankers which caught fire in the Gulf of Oman.
Mr Pompeo said US intelligence believed that the attacks were part of a wider pattern.
"Taken as a whole these unprovoked attacks present a clear threat to international peace and security, a blatant assault on the freedom of navigation and an unacceptable campaign of escalating tension by Iran," he said.
He has instructed the US ambassador to UN to raise the issue before the security council.
One of the tankers, the Front Altair, was "suspected of being hit by a torpedo", Taiwan's state oil refiner CPC Corp said.
It was on fire and adrift, according to maritime intelligence firm Dryad Global.
The other tanker - Kokuka Courageous - was attacked twice in three hours, the president of Japanese owner Kokuka Sangyo told reporters.
There were conflicting reports about what form the attacks took - a US official told Reuters that an unexploded device, believed to be a limpet mine, was spotted on the side of one of the tankers.
If confirmed, it would match the method of attack the United States believes was used last month when four other tankers were struck off the coast of the United Arab Emirates .
The crews of both tankers are believed to be safe. Staff on the Kokuka Courageous were evacuated to US Navy ship the Bainbridge.
Those on board the MT Front Altair were rescued by a passing ship and transferred to an Iranian Navy vessel.
Paolo d'Amico, chairman of the tanker association, Intertanko, said: "some 30% of the world's (seaborne) crude oil passes through the Straits.
"If the waters are becoming unsafe, the supply to the entire western world could be at risk."
All major Gulf stock markets dropped following the news, and oil prices rose by as much as 4%.
Iran's Islamic Republic News Agency (IRNA) reported that the Front Altair had sunk, but Norwegian shipping firm Frontline said it was still afloat.
Theresa May's spokesman said such attacks were completely unacceptable and the UK was ready to assist in any investigation.
United Kingdom Maritime Trade Operations - part of the Royal Navy - said it was investigating.
UN Secretary-General Antonio Guterres condemned the attacks and said the world could not afford "a major confrontation in the Gulf region".
Iran's foreign minister, Mohammad Javad Zarif tweeted that "reported attacks on Japan-related" tankers had taken place while Japanese Prime Minister Shinzo Abe was meeting Iran's Supreme Leader Ayatollah Khamenei "for extensive and friendly talks".
He added: "Suspicious doesn't begin to describe what likely transpired this morning."