SYDNEY - An Australian warship intercepted Somali pirates stalking a merchant ship in the Gulf of Aden and confiscated a cache of weapons including a grenade launcher, officials said Wednesday.

Commander Ivan Ingham said officers from his ship HMAS Toowoomba boarded the pirate ship after responding to a distress call in the Gulf on Sunday night.

A surveillance plane and helicopter sent ahead of the Toowoomba confirmed reports from the merchant ship BBC Portugal that it was being chased by a high-speed vessel full of armed men, he said.

"On reaching the suspect vessel Toowoomba launched her boarding team to investigate, search, disarm and seize," Ingham said.

"The boarding party was instructed to disarm the suspect pirates and confiscate their lethal military weapons."

A rocket-propelled grenade launcher, six AK47 assault rifles and a G3 assault rifle were taken, as well as a large quantity of ammunition.

The men said they were from Somalia but denied planning to attack the Portugal, despite being seen disposing of a ladder as the helicopter and plane approached, Ingham said.

Once the pirate ship had been cleared of weapons he said the Toowoomba confirmed it had sufficient food, water and fuel for the return journey to Somalia and ordered them out of the shipping zone.

"The quick response by HMAS Toowoomba ... ensured that the incident did not escalate into a direct attack on the merchant vessel," said Ingham.

It was the ship's first such encounter since arriving in the Gulf of Aden from the Northern Arabian Sea on an anti-piracy mission two weeks ago, he added.

Pirates have carried out more than 100 attacks in the key shipping lane that links the Indian Ocean with the Mediterranean through the Suez Canal since the start of this year.

Australia is among a raft of navies from around the world that have sent vessels to the area to combat the threat.