BANDA ACEH, Indonesia (AFP) - Three gunmen were killed in violent clashes with Indonesian security forces in Aceh, police said Thursday, jolting the westernmost province as it prepares to mark a decade of peace.

The firefight occurred just before midnight on Wednesday after police and military officers were tipped off about an armed group in Pidie, a district east of the Acehnese capital Banda Aceh.

Around a dozen gunmen opened fire on the troops, with the battle lasting an hour until the group fled on foot.

"The troops from the military and police gave chase to the remaining members of the armed group," Pidie police chief Muhajir told AFP, adding the motive for the attack remained unclear.

Officers seized an automatic machine gun and ammunition from the three victims, he said.

It's unclear who the gunmen were but violence between armed groups - often former rebels - and security forces is not uncommon in the once-restive province.

Aceh was mired in civil war for 30 years, with rebels fighting against the central government for an independent state, until a massive tsunami devastated the poverty-stricken province in 2004.

When a peace treaty was signed in August 2005, the conflict had left at least 15,000 people dead - mostly civilians - and the province with barely any functioning infrastructure.