Secretary of State Mike Pompeo has blamed Iran for “unprovoked attacks” on two oil tankers in the Gulf of Oman on June 13.
The US had made its assessment based on intelligence about the type of weapons used, he said.
Dozens of crew members were rescued after the explosions at the Japanese-owned Kokuka Courageous and the Front Altair, owned by Norway.
Both Iran and the US said they evacuated the crew.
“It is the assessment of the United States that the Islamic Republic of Iran is responsible for the attacks,” the secretary of state said at a news conference in Washington.
“This is based on intelligence, the weapons used, the level of expertise need 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.”
“This is only the latest in the series of attacks instigated by the Islamic Republic of Iran and its surrogates against American and allied interests.
“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,” Mike Pompeo said.
The blasts in one of the world’s busiest oil routes comes a month after four oil tankers were attacked off the United Arab Emirates.
No group or country has admitted the incident in May, which also caused no casualties.
The US at the time blamed Iran – but Tehran denied the accusations.
Oil prices jumped as much as 4% after the incident.
The Gulf of Oman lies at one end of the strategic Strait of Hormuz – a vital shipping lane through which hundreds of millions of dollars of oil pass.
The Norwegian Maritime Authority said that the Front Altair had been had been “attacked”, and that there were three blasts on board.
Wu I-fang, a spokesman for Taiwan’s CPC Corp oil refiner, which chartered the Front Altair, said it was carrying 75,000 tonnes of naphtha and was “suspected of being hit by a torpedo”, although this has not been confirmed.
Other unverified reports suggested a mine attack.
The ship’s owner, Frontline, said the vessel was on fire – but denied reports in Iranian media that it had sunk.
The operator of the Kokuka Courageous, BSM Ship Management, said its crew abandoned ship and were rescued by a passing vessel.
The tanker was carrying methanol and was not in danger of sinking, a spokesman said.
It is currently located about 80 miles from Fujairah in the UAE and 16 miles from Iran. The cargo remains intact.