The US Secretary of State has accused Iran of being behind attacks on two oil tankers which caught fire in the Gulf of Oman.
Mike Pompeo says the 'unprovoked attacks' are part of a campaign to escalate tension in the region.
Iranian officials, meanwhile, said the incidents were "suspicious", and took place at the same time as a meeting between the Japanese prime minister Shinzo Abe and Iran's Supreme Leader.
One of the tankers attacked is reported to be Japanese-affiliated.
Iran has previously denied involvement in incidents involving vessels in the region.
Mr Abe and other senior Japanese officials travelled to Iran in a bid to see if there's any basis for Iran-US talks to de-escalate tensions in the region.
Ayatollah Seyed Ali Khamenei has rejected the prospect of talks with the US, saying he does not consider Donald Trump "as a person worth exchanging any message with".
Mr Pompeo, meanwhile, claimed the Iranian leadership "insulted Japan by attacking a Japanese oil tanker just outside of Iranian waters".
He told reporters: "It is the assessment of the US government that Iran is responsible for the attacks that occurred in the Gulf of Oman today.
"This assessment is based on intelligence, the weapons used, the level of expertise needed to execute the operation, recent similar Iranian attacks on shipping, and the fact that no proxy group operating in the area has the resources and proficiency to act with such a high degree of sophistication."
Mr Pompeo read a statement but did not take questions or offer any evidence to support his claims.
Earlier, both US and Iranian authorities claimed they had rescued the crews on board the ships.
Details of the incidents in the Gulf of Oman emerged earlier on Thursday.
The UK's Maritime Trade Operations, which is run by the British Navy, issued an alert over an unspecified incident in the area and said it was investigating.
Taiwan’s state oil company CPC Corporation told Reuters that one of the tankers was “suspected of being hit by a torpedo.”
It said the ‘Front Altair’ was carrying 75,000 tonnes of the hydrocarbon petrochemical naptha when it was attacked at around 5am Irish time.
Maritime intelligence firm Dryad Global said the vessel is now “on fire and adrift.”
A second tanker, the ‘Kokuka Courageous,’ sustained damage to its starboard hull - causing a fire in the engine room.
It was attacked twice in three hours, according to the president of Japanese owner Kokuka Sangyo.
CPC said all crew have been rescued from the ships.
All major Gulf stock markets dropped following the news.
UN Secretary General Antonio Guterres said he noted the incidents this morning with "deep concern".
He added: "Facts must be established, and responsibilities clarified.
"If there is something the world cannot afford, it is a major confrontation in the Gulf region."
It follows US claims that Iran used mines to attack four oil tankers off the Emirati port of Fujairah last month.
Iran has denied any involvement.