The Philippines military has launched an offensive to find and 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.

Our troops are over there and the offensive and rescue operation is going on in order to end this problem

Brigadier General Romeo Tolentino

But after the deadline for a negotiated resolution expired, troops moved into the dense jungle terrain of Jolo island in the hopes of freeing the hostages by force.

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

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

On the run

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.

"Our troops are over there and the offensive and rescue operation is going on in order to end this problem," Brigadier General Romeo Tolentino, head of military forces in Jolo, said.

About 4,000 soldiers, including US-trained counter-terrorism troops, have been sent to the troubled island of Jolo to take part in the mission.

General Tolentino made no mention of the troops having actually engaged the kidnappers yet.

"We know where they are but they are still running and hiding but we will encounter them soon. Our cannons are in place and the back-up (force) is in place including helicopters," he said.

Second offensive

In tandem with the attempt to free the four Jehovah's Witnesses, the military has also mounted an attack on a separate group of suspected Abu Sayyaf militants believed to be holding three Indonesian sailors hostage on Jolo.

The Abu Sayyaf has been linked to Osama Bin Laden

The three hostages were among four Indonesian crewmen of a tugboat who were abducted by pirates on 17 June as their ship passed the island.

One of the group later escaped.

The military said the kidnappers handed their captives over to Abu Sayyaf rebels after negotiations for their release broke down.

Guerrilla killed

Troops and the suspected Muslim militants clashed in the jungles near the town Talipao, military officials said.

One guerrilla was reported to have been killed during the gun battle. No government soldiers were injured.

"There were no sightings of the hostages during the encounter," southern military commander Lieutenant-General Ernesto Carolina said.

Military commanders have been keen to stress that the crack down is not just limited to the hostage holders, but aimed at all Abu Sayyaf rebels on the island.

"The operation is against all leaders of the Abu Sayyaf group in Jolo," military chief General Roy Cimatu said.

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.