The navy said it boarded and searched the ship, which last year was caught carrying thousands of weapons and handed to Yemen's coast guard, after intercepting it in international waters in the Gulf of Oman on Tuesday.

A US guided-missile destroyer and patrol ship "interdicted the stateless vessel transiting from Iran... along a route historically used to traffic weapons to the Huthis in Yemen," the Bahrain-based US 5th Fleet said.

"US forces discovered 40 tons (36,300 kilos) of urea fertiliser, a chemical compound with agricultural applications that is also known to be used as an explosive precursor," it added.

The seizure comes at a time of high tensions in the region after a deadly drone-and-missile attack on Abu Dhabi by the Huthis prompted the Saudi-led coalition to launch air strikes on Yemen this week.

The pro-government coalition acknowledged strikes on Sanaa and Hodeida that killed at least 17, including children, and triggered an internet blackout across the impoverished country.

But it denied bombing a prison facility in Saada, the rebels' northern heartland, which killed at least 70 and wounded more than 100 others.

The same "stateless fishing vessel" was found to be carrying thousands of AK47 assault rifles, rocket-propelled grenade launches and other weapons when it was stopped last February, the US Navy said.

The Saudi-led coalition and its allies, including the United States, regularly accuse Iran of providing military support to the Huthis, claims that Tehran denies.