Manila  - Muslim militants on a southern Philippine island would face a full military assault if they carry out a threat to behead one of three Red Cross hostages at the end of the month, a military spokesman said Thursday.

Muslim Abu Sayyaf rebels made the threat to force government troops to move further away from their encampment in the jungles of Indanan town on Jolo island, 1,000 kilometres south of Manila.

Lieutenant Colonel Edgard Arevalo, a navy spokesman, said the Abu Sayyaf's demand was an "impossible condition" as it would mean government forces abandoning more than half of their positions on the island and leaving communities unprotected.

Arevalo said punitive actions would definitely be launched against the guerrillas once they harm the hostages - Swiss Andreas Notter, Italian Eugenio Vagni and Filipino Mary Jean Lacaba.

"It may sound a threat, but actually it isn't," he said. "It is but a logical consequence of what they will do."

Arevalo noted that the military has been holding off any assault on the Abu Sayyaf and allowing negotiations to ensure the hostages' safety.

"But if the hostages are harmed and are killed, they'll take away the reason for us to negotiate with them," he said.

The hostages, staff of the International Committee of the Red Cross (ICRC), were abducted on January 15 after visiting the Jolo provincial jail to oversee a water and sanitation project.

Last week, clashes erupted between the Abu Sayyaf and Marines circling the kidnappers, killing three government troops and six guerrillas.

Abu Sayyaf leader Albader Parad threatened to behead one of the hostages if the military continued its offensive or launched a rescue. He also promised to free one of the Red Cross workers if the troops moved away from their encampment.

While the military repositioned its forces, the rebels did not free a hostage and demanded for a larger pullout.

The new threat to behead was issued on Monday as the military refused to move further away and stepped up a blockade to prevent food and other supplies from reaching the guerrillas.

The Abu Sayyaf has been blamed for some of the worst terrorist attacks and high-profile kidnappings in the Philippines. In the past, the rebels have beheaded hostages, such as an American tourist in 2001, when the government refused to give in to their demands. (dpa)