The US Navy said Saturday it had seized one million rounds of ammunition along with rocket fuses and propellant being smuggled on a fishing trawler from Iran to war-torn Yemen.

The cargo was discovered on Thursday "during a flag verification boarding", the Bahrain-based United States Fifth Fleet said in a statement, noting it was the "second major illegal weapons seizure within a month" along the maritime route.

"This significant interdiction clearly shows that Iran's unlawful transfer of lethal aid and destabilising behaviour continues," Vice Admiral Brad Cooper said.

"US naval forces remain focused on deterring and disrupting dangerous and irresponsible maritime activity in the region."

The Iran-backed Huthi rebels seized the Yemeni capital Sanaa in 2014, prompting a Saudi-led coalition to intervene the following year.

Since then, a grinding war has killed hundreds of thousands and pushed the impoverished nation to the brink of famine.

A UN-brokered ceasefire that took effect in April brought a sharp reduction in hostilities. The truce expired in October, though fighting largely remains on hold.

The fishing trawler intercepted on Thursday was carrying nearly 7,000 rocket fuses and "over 2,100 kilogrammes of propellant used to launch rocket propelled grenades", the statement said.

"The direct or indirect supply, sale or transfer of weapons to the Huthis in Yemen violates UN Security Council Resolution 2216 and international law," it added.

Last month the US navy said it had scuttled a boat transporting "explosive materials" from Iran to supply the Huthis, with enough power to fuel a dozen ballistic rockets.