The United States is ready to provide "assistance" to end the hostage crisis in Sulu province involving two European Red Cross volunteers, officials said Thursday.

"We stand ready to help our Philippine counterparts with whatever they might request of course," Thomas Gibbons, the US embassy's deputy envoy and political affairs counsellor, told reporters at the opening ceremony of annual joint US-Philippine military manoeuvres.

Gibbons, however, said US forces would not take part in any direct combat missions.

About 6,000 US troops and some 2,500 Filipinos are involved in two-week Balikatan (Shoulder to Shoulder) exercises, which include field training exercises and humanitarian missions in the main island of Luzon and eastern Bicol region.

However, a small number of US forces have also been carrying out various missions for several years on the southern island of Jolo, where Abu Sayyaf militants have been holding Italian Eugenio Vagni and Swiss national Andreas Notter since January 15.

A third hostage, Filipina Mary Jean Lacaba, was freed earlier this month.

The rebels have been driven deep into the Jolo jungle, where their supplies are running out while they are locked in a tense stand-off with troops.

While the Philippines and the US are bound by a decades-old mutual defence treaty, foreign troops are prohibited from combat operations on domestic soil, Filipino military chief General Alexander Yano said.

He noted that the US military had trained and equipped Filipino special forces against the Abu Sayyaf in the past, and the local troops were now capable of dealing with the insurgents on their own.

However, if there is a need for assistance, the Philippines may ask the US forces to provide "technical intelligence", medical air evacuations and airlifting of equipment and transport.

"Other than that, as in the past, we have not utilised them for any direct combat action," Yano said.

On Wednesday, the local crisis negotiating team on Jolo dispatched five Muslim clerics to the Abu Sayyaf lair in a last-ditch effort to convince them to hand over the hostages peacefully.

Yano said there had been no update from the intermediaries and declined to comment on the next move. He has, however, said troops can launch a rescue operation once given the go-ahead by a crisis committee on the ground headed by the provincial governor of Jolo.

The Abu Sayyaf is a small gang of self-styled Islamic militants blamed for the Philippines' worst terrorist attacks, including deadly bombings and kidnappings.

The US government has placed it on its list of foreign terrorist organisations.

Two US hostages seized from an island resort in 2001 were killed while in captivity, one of them beheaded by the Abu Sayyaf.