The Philippines military is poised to mount an offensive to free four Jehovah's Witnesses being held hostage by gunmen believed to have links to Muslim rebels the Abu Sayyaf.

The kidnappers, thought to be led by a nephew of Abu Sayyaf leader Radulan Sahiron, had been given until Tuesday to free the female hostages.

The president has instructed us to go all out, get them and so that's what we're going to do

Defence Secretary Angelo Reyes

However, officials from the Philippines military said that they had received no response from the guerrillas and therefore a military assault was the next logical step.

The four were part of a group of eight abducted on the southern island of Jolo last week.

Two Muslim members were freed, but the severed heads of two other hostages were found on Thursday.

The Abu Sayyaf is best known for kidnapping for ransom, though the US and Philippines governments link the group to Osama bin Laden's terrorist network.

Jungle hideout

"The negotiators received no definite word from the kidnappers and instead they went deeper into the jungle," southern Philippine military commander Lieutenant General Ernesto Carolina said.

About 4,000 soldiers, including US-trained counter-terrorism troops, have been sent to root out the extremist Muslim militants in Jolo.

But analysts warn that Jolo island is home to several armed groups with sometimes shifting loyalties. This, combined with the jungle terrain, may make the guerrillas difficult to track down.

Jolo army chief Brigadier General Romeo Tolentino said the gunmen and their captives were believed to be constantly moving in dense jungle to evade troops.

"Our troops are combing the jungle inch for inch, but the kidnappers are constantly on the run," he said.

"Rescue operations will continue. This is a parallel effort by the military to pressure the kidnappers to free the hostages without any condition of ransom," he added.

Ongoing negotiations

Local negotiations to bring about a peaceful resolution to the crisis are still continuing.

Yusop Jikiri, governor of Sulu province which contains Jolo, said another group had offered to negotiate with the kidnappers.

The US believes the Abu Sayyaf has links to Osama Bin Laden

"We hope it will succeed, but we are poised and we will conduct operations against these kidnappers," Defence Secretary Angelo Reyes said.

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.

The latest incident is a grim reminder that the Abu Sayyaf, which Washington has linked to Osama bin Laden's al-Qaeda terror network, remain a danger.