Somalian pirates occupying a hijacked Ukrainian ship loaded with arms refused to back down from their $20-million US ransom demand Thursday and said they will fight back against any commando-style rescue attempts.

A half-dozen U.S. navy warships have surrounded the cargo ship MV Faina, which the pirates seized last Thursday off the coast of Somalia as it transported 33 Soviet-designed tanks and heavy weapons to the Kenyan port of Mombasa.

The group will not release the ship for less than the $20 million it had originally demanded, pirate spokesman Sugule Ali told the Associated Press in a satellite phone interview from the Faina.

"We would never reduce the ransom," he said.

The pirates and the ship's owners have been negotiating over the ransom demand.

Somalia OK's use of force

The Somalian government on Wednesday authorized foreign powers to use whatever force is necessary to free the ship from the pirates.

Asked about fears that a foreign country might attack, as French commandos have done in the past to free hijacked ships, Ali insisted his pirates will fight back.

"Anyone who tries to attack us or deceive us will face bad repercussions," he said.

Ali also distanced himself from reports quoting a leader of Somalia's Islamic insurgency who urged the pirates to destroy the ship if they are not paid.

"We have nothing to do with insurgents or terrorist organizations, we only need money," Ali said, adding that a plan was in place to release the ship and its crew of 20 once the ransom is received.

Moscow has sent a warship to protect the few Russian hostages on board the Faina, but it will take several more days to arrive.

The Russians have used commando tactics to end several hostage situations in the past, but scores of hostages have died in those efforts.

Some 26 ships have been hijacked off the notorious African waters this year.

The Faina case, the most high-profile hijacking off Somalia's lawless coast, has highlighted how the country's increasingly brazen pirates are drawing the concern of global superpowers along one of the world's busiest shipping lanes.