US troops are helping Philippine navy forces search for the body of a key leader of the Muslim rebel group Abu Sayyaf, who is thought to have died in a gun battle.

Philippines and US officials are convinced that Abu Sabaya was killed in a shoot-out at sea on Friday.

Abu Sabaya: Notorious spokesman for the rebels

But three days of searching have failed to produce any trace of him or two of his followers who are also missing.

Two captured rebels, speaking on a video released by the military, have said their leader was shot in the back during the clash, which took place off southern Zamboanga del Norte province.

"The firing was at close range. I'm sure [he is dead]," said one of the men, Aburakman Ismael.

Two other men were also captured, one of whom later died of heart failure while in custody.

US confidence

Local media have expressed scepticism about whether Abu Sabaya really is dead. But the military has said his body may have been swept far out to sea or even eaten by sharks.

Brigadier General Donald Wurster, commander of US troops helping train Philippine soldiers, said US Navy SEAL commanders were helping in the search. However he said they would not be bringing in expensive salvage ships.

"I don't think it's worth that investment to recover the bodies of criminals that we know are dead," he said.

Dozens of fishermen have also joined the search after the military announced a $1,000 (50,000 peso) reward for anyone who found Abu Sabaya's body.

Abu Sabaya's death would be a major blow to the rebel group, which is notorious for carrying out kidnaps for ransom.

Hostages killed

Hostage-taking is the group's main activity although its stated aim is the creation of an independent Muslim state in the south of the mainly Christian Philippines.

The US believes the group has links with Osama Bin Laden's al-Qaeda network. In May it put a $5m bounty on the head of Abu Sabaya, along with four other leaders.

Abu Sabaya is the group's main spokesman and is notorious for calling local radio stations on his satellite phones to taunt the authorities.

At the time of the shoot-out he was on the run following a shoot-out with Philippine troops in which a US hostage and a Filipina nurse he and his men had held hostage for more than a year were killed.

The group is led by the reclusive Khadafi Janjalani, the brother of founder Abdurajak Janjalani who was killed by troops in 1998.