The ceasefire in Aceh's separatist war looked fragile after Indonesian police shot dead three rebels in the seventh armed clash in a week.

Security forces and the Free Aceh Movement (GAM) have yet to agree on a demilitarisation plan for the province, almost two months after a crucial phase of the December 9 peace pact began.

Regular police and Brimob paramilitary police killed three GAM members Thursday after shots were fired while they were checking identity cards at a roadside kiosk at Lueng Sagoe in Pidie district, said local police chief Adjunct Chief Commissioner Maryanto.

Maryanto said no one was hurt in the initial shooting at police, who killed three GAM members when they fired back. Two pistols were found on the bodies.

A GAM spokesman, Anwar Husen, said two GAM members had pistols but denied any attack on police. He called the shooting a serious violation of the peace pact.

The military said an air force sergeant was kidnapped Thursday in Aceh Besar district by suspected rebels. GAM denied responsibility.

A Swiss-based mediation group, the Henry Dunant Centre (HDC), said there had been seven armed clashes over the past week along with extortion and reports of the "disappearance" of civilians.

David Gorman, the HDC representative in Aceh, urged the Joint Security Committee (JSC) which monitors the truce to investigate all incidents. The JSC is made up of representatives from the security forces, the rebels and the HDC. It has some 150 monitors in the field.

Gorman told AFP one team was harassed in West Aceh last week and has been temporarily withdrawn. "I think this is a big challenge for the JSC but both parties recognised that when they entered into the agreement there would be problems," he said.

The number of killings has declined dramatically since the agreement, the first to be monitored by foreigners, took effect.

But each side in the separatist war, which has claimed an estimated 10,000 lives since 1976, accuses the other of violations. Jakarta says the rebels are spreading lies that the peace deal will lead eventually to independence.

Indonesian military commander General Endriartono Sutarto said Thursday that GAM was practising extortion and attacking troops. It had also failed to lay down any weapons and was still demanding independence.

"In these circumstances, how can we continue the truce?" he said, while adding any decision to go back to war was up the government.