US Secretary of State Mike Pompeo has blamed Iran for “unprovoked attacks” on two oil tankers in the Gulf of Oman.
He said the US had made its assessment based on intelligence about the type of weapons used.
A senior Iranian official earlier told the BBC “Iran has no connection” with the explosions on Thursday morning.
Dozens of crew members were rescued after the blasts on the Japanese-owned Kokuka Courageous and the Front Altair, owned by Norway.
The explosions 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 Thursday’s 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.
What did Mike Pompeo say?
“It is the assessment of the United States that the Islamic Republic of Iran is responsible for the attacks,” the US 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,” Mr Pompeo said.