JAYAPURA, Indonesia -- Security forces have located the bodies of 16 people and a soldier who were killed in one of the bloodiest separatist attacks in Indonesia's restive Papua province, a military official said Thursday.

Army helicopters transported eight bodies and eight survivors, including a 4-year-old boy, from a remote mountainous village in Nduga district, Papua province military spokesman Col. Muhammad Aidi said. Gunfire between security forces and an armed group linked to the Free Papua Movement was hampering efforts to recover the eight other bodies, he said.

The area is a stronghold of separatists who have battled Indonesian rule in the impoverished region for half a century.

Police earlier said 31 workers and a soldier were killed Sunday when gunmen stormed a government construction project in a remote village in Nduga district, citing reports from witnesses.

Authorities on Wednesday revised the figure to 19 civilians, including workers, and a soldier, based on the accounts of survivors.

Security forces on Tuesday rescued 12 survivors, including five injured construction workers.

A forensic team at a hospital in the mountain mining town of Timika was determining whether the eight bodies were those of other construction workers employed by PT. Istaka Karya, a state-owned construction company, to build bridges on a section of the trans-Papua road network that will connect cities and districts in the province.

Indonesia's government, which for decades had a policy of sending Javanese and other Indonesians to settle in Papua, is now trying to spur economic development to dampen the separatist movement.

The workers are considered outsiders by the separatists.

"We are still searching for the bodies of other civilians possibly killed by the armed group," Aidi told The Associated Press.

He said the body of a soldier who was shot Monday when an armed group attacked a military post in Mbua village in the same district has been flown to Sorong in neighbouring West Papua province for a military funeral.

A spokesman for the West Papua National Liberation Army, the military wing of the Free Papua Movement, could not be reached for comment.

Indonesian President Joko "Jokowi" Widodo expressed his condolences to the families of the victims on Wednesday and ordered the military and police to arrest the perpetrators of the worst separatist attacks during his administration.

"There is no place for these armed criminal groups in Papua or the rest of the country," Widodo said at the presidential palace in Jakarta.

He said the attack would not dissuade his government from continuing to develop Papua, including the construction of the 4,600-kilometre (2,875-mile) trans-Papua road, which his administration has claimed was widely supported by local people.

The road, which will stretch from Sorong in West Papua province to Merauke in Papua province, is expected to be completed next year and help boost economic development in both provinces.

Papua, a former Dutch colony in the western part of New Guinea, was incorporated into Indonesia in 1969 after a U.N.-sponsored ballot that was widely seen as a sham. A small, poorly armed separatist group has been battling for independence since then.

The low-level insurgency has plagued the mineral-rich region, which is ethnically and culturally distinct from much of Indonesia.

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Associated Press writer Niniek Karmini in Jakarta contributed to this report.