Somewhere on this 20-square-mile speck in the Philippines archipelago, a pair of American missionaries is being held captive by a terrorist group linked to Usama bin Laden.

And as it has been for the last eight months, the nightly mission of the Philippine marines on patrol here remains the same: Find Martin and Gracia Burnham, and find them alive.

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Steve Harrigan in the Philippines

The Burnhams, missionaries from Kansas, are being held by the Abu Sayyaf group. The group is said to have close links with Al Qaeda, and may be among the next targets in the U.S. war against terrorism.

The strategy of the Philippine government is to force the terrorists on to smaller and smaller pieces of land. On this day, these Philippine Marines waited in ambush for the terrorists to walk the river.

The heat, the mosquitoes, the danger make it a tough job. But morale in the 5th Marine Battalion remains high.

"When was the last time you saw anyone from Abu Sayyaf?" Fox News asked one Marine. "August," came the reply.

Yet despite no terrorist sightings in five months, they labor on, by foot, with the knowledge that American special forces, armed with night vision and helicopters, may soon be joining the fight.

Several hundred U.S. troops have arrived or are on their way to the Philippines, with the aim of training and supplying local units in their fight against Aby Sayyaf and other terrorist groups. It’s not exactly clear what role the U.S. will play in actual operations against such groups, though at least some of the troops headed here are special forces.

Meanwhile, there is scant information on the welfare of the Burnhams. They last appeared in a videotape made in November. The broadcast was the first to show the Wichita couple since they were abducted May 27 while celebrating their 18th wedding anniversary at a resort.

The couple, who are in their early forties, said on the tape they had developed mouth sores from lack of nutrients, and rely on scarce supplies of cassava and bananas.

Martin Burnham, who was heavily bearded, has lost weight. He appeared composed as he expressed concern about their three children, Jeffrey, 14, Melinda, 12, and Zachary, 10, who are being cared for in Kansas by their grandparents.

Abu Sayyaf is the smaller of the Islamist groups fighting to establish an Iranian-style Islamic state in Mindanao, an island in the southern Philippines. The group, whose name means, "Bearer of the Sword," split from the Moro National Liberation Front in 1991.

Although based almost exclusively in the southern islands, Abu Sayyaf is believed to have ties to a number of fundamentalist organizations around the world, beyond Al Qaeda. The leader of the group, Abduragak Abubakar Janjalani, is a veteran of the war in Afghanistan.

The group has a membership of approximately several hundreds of young Islamic radicals, many of whom were recruited from universities and high schools.

The Associated Press contributed to this report.

Steve Harrigan currently serves as a Miami-based correspondent for Fox News Channel (FNC). He joined the network in 2001 as a Moscow-based correspondent.