Jakarta (Agencies) – Hundreds of people besieged a peace monitoring office in the restive province of Aceh on Monday, one day after a separate facility was torched, in the latest blow to a landmark peace pact that is threatening to unravel.

The Geneva-based Henry Dunant Centre, which brokered the peace deal between the Freedom Aceh Movement (GAM) rebels and Jakarta in December, said the incident took place late on Monday when about 750 people demanded the monitoring team leave the town of Tapaktuan in south Aceh. It was unclear what had angered the crowd.

"They made a demand that the monitoring team leave by Friday and they wanted a letter immediately to that effect," Steve Daly, a spokesman for the Henry Dunant Centre, told Reuters.

He said there was an implied threat in the demand, prompting the monitors to issue the letter, although they were still in the town. Daly said no decision had been made on withdrawing the 12 monitors, including two Thais. The crowd had since disbursed.

On Sunday, 300 Acehnese set fire to a peace monitoring office in the East Aceh town of Langsa. No one was hurt but it followed the ransacking of another office a month ago.

Antara reported on Monday that thirteen people have been interrogated following the torching of a peace monitoring office in the troubled province of Aceh on Sunday.

"Four people have confessed vandalism and torching for disappointment towards separatist Free Aceh Movement [GAM] member in the East Aceh chapter of the Joint Security Council," National Police Chief General Da'i Bachtiar said here on Monday.