Jakarta – Three Indonesian policemen and a separatist rebel were killed when gunmen attacked a police truck in the restive province of Aceh, reports said Monday.

Police said the truck was ambushed despite a three-month truce. The Aceh Merdeka separatist movement (GAM) said it was a shootout when their men inadvertently met with the policemen, the Banda Aceh-based Serambi daily said.

"The ambush and firing by the GAM clearly violates the rules that were jointly agreed between the Indonesian Republic and the GAM," Aceh deputy police chief Colonel Teuku Asikin told the daily.

The truce, signed in Geneva in May and which came come into effect on June 2, calls for both sides to attempt to reduce tension and violence in Aceh by confining their troops and laying down their weapons.

Asikin said the police truck was attacked as it passed Blang Karieng village on Sunday. Grenade-launchers were used and the truck was fired on, he said. Three policemen were killed, six injured and a GAM member was shot dead, Asikin said.

The GAM deputy commander of the Pasee region, Tengku Sofyan Daud, told Serambi the incident was not intentional but was unavoidable. "We were caught by the Brimob (the police mobile brigade) as we were trying to evade pursuit by members of the sub-district military command in Buloh Blang Ara," Daud said.

The leader of the Indonesian delegation to the joint committee on security modalities set up to implement the truce, Colonel Ridwan Karim, said he will lodge a protest with GAM over the ambush, Serambi said. "This is a serious violation that needs to be followed up," Karim said, adding that protests would also be lodged with the truce monitoring team and the joint committee in Geneva.

The truce was brokered by the Henry Dunant Center in Switzerland, and is being supervised by teams from both sides.

In another incident Sunday some 10 gunmen ambushed a motorcycle convoy of 20 soldiers in Cot Trieng Paloh in North Aceh, leaving one soldier injured, North Aceh police chief Superintendent Syafei Aksal told Serambi.

GAM has been fighting since the mid-1970s for an independent Islamic state. Sympathy among the population for GAM has been fanned by decades of tough military action against the rebels, and by resentment against Jakarta for draining the province's rich natural resources, which include natural gas.