Jakarta – More than 1,000 Indonesian troops have tightened their siege of a separatist rebel group in Aceh and are ready to attack if necessary, AFP reported.

Troops have moved closer to the Free Aceh Movement (GAM) rebels but are delaying any attack for fear of hurting civilians who are with the rebels, saidLieutenant Colonel Firdaus Komarno.

"Until now we are still trying to call on them to surrender," he said, adding that any order to attack would depend on the situation in the area.

The troops have encircled an estimated 40-50 GAM members who are trapped in a mountainous nine-square-km. Army posts have been setup every 100 metres. Top officers including Military Chief for General Affairs Lt. Gen. Djamari Chaniago overflew the area Friday by helicopter.

The 15-day siege, during which troops have used helicopter-launched rockets, mortars and light tanks, is the biggest operation for months against GAM which has been fighting for an independent state since 1976.

The siege was made after GAM postponed the signing of a truce between it and the government, which supposed to be occured earlier this month.