Banda Aceh – International peace monitors in Indonesia's Aceh province yesterday said they would withdraw from the south of the province after a mob of up to 900 people threatened to burn down another of the group's offices.

The incident is the latest blow to a fragile peace between separatist rebels and the government in Aceh province aimed at ending a 26-year-old war in the region. On Sunday, a pro-Jakarta mob torched an office belonging to the Henri Dunant Centre, the Swiss-based non-governmental organisation which is overseeing the peace deal. It was the second time an office belonging to the observers had been attacked since the deal was signed in Geneva on December 9.

A spokesman for the centre, Mr Steve Daly, said an "organised mob" of around 900 people yesterday surrounded an HDC office in Tapaktuan in south Aceh, 320km south-east of the provincial capital of Banda Aceh.

He said the protesters threatened to burn down the building unless the monitors left the region by Friday. He said it was unclear who was behind the incident.

After talks with the mission's leadership in Banda Aceh, staff members handed the demonstrators a letter agreeing to leave South Aceh by Thursday, he said. The crowd, which carried banners accusing the observers of being biased towards the rebels, then dispersed, witnesses said.

Rebels and some rights activists have accused elements of the military of attempting to destroy the peace process by orchestrating the violence and intimidation – a charge denied by police and military officials.

On Sunday, Mr Daly said the peace process was "clearly in trouble" as a result of the attack, adding that yesterday's incident only served to reinforce those fears.

In recent weeks, the government and rebels have repeatedly accused each other of not abiding by the terms of the peace deal. However, the deal has succeeded in dramatically stemming bloodshed in the province.

Insurgents have been fighting since 1976 for an independent state in oil and gas-rich Aceh. About 12,000 have been killed in the conflict in the past decade.