We expect the Abu Sayyaf will be neutralised

Defence Secretary Angelo Reyes

The US has linked the Abu Sayyaf to Osama Bin Laden's al-Qaeda network. Correspondents say the deployment is the first significant military expansion of Washington's so-called war on terrorism beyond Afghanistan.

An initial group of American advisors is already in the city of Zamboanga, and the remainder will start arriving on Tuesday.

'Shoulder-to-shoulder'

The troops, including 150 special forces, will join 1,200 Philippines soldiers in the exercise called Balikatan - shoulder-to-shoulder - on the southern island of Basilan, an Abu Sayyaf stronghold.

The US links the Abu Sayyaf with al-Qaeda

Foreign troops are forbidden by Philippine law from joining combat operations in the country.

Philippine Defence Secretary Angelo Reyes said: "The US troops will be armed for self-protection and will engage in combat only for self-defence."

The exercise is scheduled to last six months but Mr Reyes said it could be extended to the end of the year.

"We expect that the Abu Sayyaf will be neutralised and that [the] Burnham couple shall have been recovered," he said.

Martin and Gracia Burnham from Wichita, Kansas, were snatched from a western Philippines tourist resort in May along with about 17 Filipinos who have since been killed or released for ransom.

Possible casualties

A military spokesman said the Americans were aware of the risks involved and had already "factored in " the possibility that they might sustain casualties.

The Philippines hopes to free the Burnhams soon

The Abu Sayyaf says it is fighting for a separate Islamic state, but kidnapping has been its main activity.

The Burnhams were taken to the Abu Sayyaf stronghold of Basilan island, where they are being held with a Filipina nurse, Deborah Yap.

A third American captured with them, Guillermo Sobero from California, was found beheaded.

Presidential spokesman Rogoberto Tiglao said the Abu Sayyaf were now regarded as Manila's biggest security problem.

"A very important part of our stability would be to totally get rid of the Abu Sayyaf," he said.