Nairobi - Almost 2 000 assault rifles plus grenade launchers and heavy machine guns were seized by an Australian warship, the navy said on Monday, part of international efforts to police the seas off Somalia.

Sailors from Australia's HMAS Darwin boarded the fishing boat that was "headed towards the Somalia coast" some 313km off Oman, seizing a huge cache of embargo-busting weapons hidden under fishing nets, the navy said.

"The weapons were seized under United Nations sanctions which authorise interdiction on the high seas of illicit weapons destined for Somalia," Australian Vice-Admiral David Johnston said, adding that, "such a large haul of illicit weapons is highly significant."

A total of 1989 AK-47 assault rifles, 100 rocket propelled grenade launchers, 49 PKM machine guns, and 20 mortar tubes were seized.

No details were given as to which group may have ordered the weapons believed to be heading for Somalia, a still war-torn nation riven by conflict between multiple militia forces, including the al-Qaeda linked Shabaab, as well as a national army and regional millitaries.

If it had not been stopped, the boat would also have passed by the coast of Yemen, also in civil war.