An Indonesian military helicopter crashed on Sunday in Central Sulawesi province, killing all 13 people on board, an armed forces spokesman said.
The helicopter was carrying personnel to reinforce a police operation against Islamist militants operating in the district of Poso, military spokesman Tatang Sulaiman said.
"We suspect that bad weather caused the crash," he said.
Poso was the scene of deadly Muslim-Christian violence in 1999-2000. The area has since become a known hotbed of militant activity.
Last week police killed two suspected Islamist fighters from China's Uighur ethnic group in a gunfight in Poso as part of an operation to crush a group led by a wanted militant named Santoso.
More than 2,500 military and police personnel have been deployed in the area since January.