US Secretary of State Mike Pompeo has blamed Iran for "unprovoked attacks" that damaged two oil tankers in the Gulf of Oman.
He told a news conference on Thursday the US had made its assessment based on intelligence about the type of weapons used.
It was also based on recent incidents in the region that the US also blames on Iran, including a similar attack on tankers in May and the bombing of an oil pipeline in Saudi Arabia by Iranian-backed fighters, he said.
Mr Pompeo added that the ships were targeted as part of a "campaign" of "escalating tension" by Iran and a threat to international peace and security.
Dozens of sailors were evacuated after explosions on the MT Front Altair and the Kokuka Courageous.
The US Navy said one oil tanker was adrift and on fire, and it was assisting the tankers after receiving two distress calls.
Each was loaded with petroleum products, and the Front Altair burned for hours, a fire that charred half of one of the vessel's sides and sent up a column of thick, black smoke.
Mr Pompeo told reporters this afternoon: "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.
"It is the assessment of the United States government that the Islamic Republic of Iran is responsible for the attacks that occurred in the Gulf of Oman today."
He added: "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 sophisticatiom."
The suspected attacks occurred about 40 kilometres (25 miles) off the southern coast of Iran.
The Norwegian-owned Front Altair, loaded with naphtha from the United Arab Emirates, radioed for help as it caught fire.
A short time later, the Japanese-owned Kokuka Courageous, loaded with methanol from Saudi Arabia and Qatar, also called for help.
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The US Navy sent a destroyer, the USS Bainbridge, to assist, said Cmdr Joshua Frey, a 5th Fleet spokesman. He described the ships as being hit in a "reported attack," without elaborating.
Frontline, the firm that operates the Front Altair, told The Associated Press that an explosion was the cause of the fire.
Its crew of 23 - from Russia, the Philippines and Georgia - was safely evacuated to the nearby Hyundai Dubai vessel, it said.
BSM Ship Management said the Kokuka Courageous sustained hull damage and its 21 Filipino sailors had been evacuated, with one suffering minor injuries.
All 21 were placed aboard the Bainbridge, according to Lt Col Earl Brown, a spokesman for the US Central Command.
Earlier, Iranian state television said 44 sailors from the two tankers were transferred to an Iranian port in the southern province of Hormozgan.
Downing Street called the suspected attacks "completely unacceptable" and said the UK is ready to assist in any rescue or investigation.
Theresa May's official spokesman told a Westminster briefing: "Attacks on civilian vessels like these are completely unacceptable.
"It's vital that freedom of navigation is respected and observed.
"We are in contact with international partners and are ready to assist in any rescue effort or investigation."
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