Trump blasts Iran on Fox & Friends as tensions soar over Gulf of Oman attack

"They're not going to be closing it," he said in an interview on Fox News television.

Speaking hours after the US military released grainy footage it said showed an Iranian patrol boat removing an "unexploded limpet mine" from one of the tankers, Trump said "Iran did do it."

"You know they did it because you saw the boat," Trump told the "Fox and Friends" show.

"I guess one of the mines didn't explode and it's probably got essentially Iran written all over it."

"You saw the boat at night, successfully trying to take the mine off -- and that was exposed," he added.

Iran rejects the US accusations.

Iranian Foreign Minister Mohammad Javad Zarif tweeted that the US had "immediately jumped to make allegations against Iran without a shred of factual or circumstantial evidence."

He accused Washington of seeking to "sabotage diplomacy" as Japanese Prime Minister Shinzo Abe visited Iran. One of the targeted vessels is owned by a Japanese company while the other was Norwegian-operated.

Iran has repeatedly warned in the past that it could block the Hormuz Strait in a relatively low-tech, high-impact countermeasure to any attack by the far more powerful United States.

Doing so, would disrupt traffic of oil tankers between the Persian Gulf and the Gulf of Oman, which leads to the Indian Ocean and global export routes.

"If the hostility of enemies increase, we will be able to do so," Iran's armed forces chief of staff, Mohammad Bagheri, told semi-official ISNA in April.

Trump played down the threat.

"It's not going to be closed, it's not going to be closed for long and they know it. They've been told in very strong terms," Trump told Fox News.