A mob of some 1,000 people ransacked and torched a peace monitors' office in the troubled Indonesian province of Aceh, witnesses and a staff member said.

The mob arrived at the empty Tripartite Monitoring Team office in Langsa, the district town of East Aceh, in trucks and pickups around 9am and forced their way in, destroying computers and other equipment before setting the building on fire, an eyewitness told journalists there. The witness said there were no police on site.

The protesters, who were demanding the release of hostages allegedly captured and held by the separatist Free Aceh Movement (GAM), then went to the local district military command, which is near a hotel where the members of the monitoring team are staying.

They were also demanding the disbanding of the Joint Security Committee (JSC), which supervises the implementation of a December 9 peace pact between the government and the GAM.

The JSC comprises representives from the government, the GAM and from the Geneva-based Henry Dunant Center (HDC) which has facilitated peace talks since 2000. The JSC also oversees the tripartite monitoring teams.

"We condemn this incident and we call on the Indonesian government to provide security for the members of the monitoring team. We also call on the Indonesian authorities to conduct a full investigation and bring the perpetrators to court," HDC spokesman David Gorman told AFP.

He said that the JSC has already received a report of the incident and was currently gathering more information. The members of the monitoring team in Langsa had been moved to the military compound for their own safety, he added. The identity of the mob was unclear.

Past attacks on JSC and monitoring teams' offices have been conducted by pro-government groups, including what the GAM said were local militias nurtured by the Indonesian military. The military has denied the charges.

Two members of the Tripartite Monitoring Team were injured in an attack on the JSC office in Takengon, Central Aceh on March 3. Aceh's separatist war has claimed an estimated 10,000 lives since 1976.