The Philippine army is to give civilian officials time to negotiate the release of four Jehovah's Witnesses taken hostage on the southern island of Jolo on Tuesday.

Military chief General Roy Cimatu said the hostage-takers, believed to be Muslim guerrillas belonging to the Abu Sayyaf group, had five days to hand over their four female captives.

The severed heads of two other hostages were found on Thursday, an act the US condemned as a "terrorist atrocity".

General Cimatu said more than 1,000 extra troops were on their way to Jolo and would join the hunt for the guerrillas if they did not hand over the remaining hostages.

Some of the soldiers being sent received training from the United States military earlier this year, as part of the US war on terrorism.

The US and the Philippines regards the Abu Sayyaf as terrorists with possible links to Osama bin Laden, though the group is better known for taking hostages for ransom.

The latest kidnapping is believed to have been led by a nephew of Abu Sayyaf leader Radulan Sahiron.

But analysts warned that Jolo island was home to several armed groups with sometimes shifting loyalties, and it was not clear if the latest incident was motivated by ideology or profit.

Setback

The US troops left less than a month ago

Either way, the hostage taking and beheading marked a severe setback for the Philippines' security forces.

Only a few weeks ago American and Philippine officials said the army had destroyed the Abu Sayyaf and its base on nearby Basilan island had been wiped out.

Their claim to have killed Abu Sabaya, a prominent Abu Sayyaf leader, was also cast into doubt on Friday when a Christian priest told a local newspaper that the man had been seen alive.

The hostages, who mainly came from the southern Philippine island of Mindanao, were abducted on the outskirts of Jolo town along with two Muslims. The Muslims were later released.

Jolo's army chief, Brigadier General Romeo Tolentino, described the kidnappers as "terrorists, animals and barbarians".

He said a note was attached to one of the heads, saying: "Those who do not believe in Allah will suffer the same fate."

BBC correspondent John Mclean says this latest abduction appears to be a message to the government that the Abu Sayyaf is far from finished.