The Philippine government said it was impossible to comply with the latest demand of kidnappers holding three Red Cross workers to empty five southern towns of security forces by Monday night.

The al Qaeda-linked Islamic militant group Abu Sayyaf imposed the demand when it contacted government negotiators late on Sunday night, threatening to behead one of three hostages by 2 p.m. on March 31, Interior Secretary Ronaldo Puno said.

"I don't know how they can be serious if they are saying that we should evacuate all of these areas within 24 hours and then schedule a beheading at 2 p.m. tomorrow," Puno told reporters in Manila.

"Frankly, we are very disheartened by these new demands, which were given late last night, and which clearly has no physical possibility of being complied with."

The government complied earlier this month to two demands to move back the security cordon around them. But Puno said those decisions were not a sign of Manila's weakness in dealing with the militants.

"Our highest priority is the safety of the three hostages," Puno said as he appealed to the rebel leaders to rethink their new demand and the March 31 deadline.

By Sunday, nearly 1,500 soldiers, police officers and armed civilian volunteers had moved back by about 15 km (9 miles) from the rebels' position in the interior of southern Jolo island.

Now the rebels want the government to move all security forces out of five towns in the west of the island where the Abu Sayyaf are, Puno added.

The rebels have been holding Swiss national Andreas Notter, Italian Eugenio Vagni and Filipina Mary Jean Lacaba -- workers of the International Committee of the Red Cross -- since Jan. 15.