A small plane contracted by the Pentagon crashed in the southern Philippines on Thursday, killing all four people aboard, including a member of the U.S. military, officials said.

The plane, a Beechcraft King Air 300, crashed in the municipality of Ampatuan in Maguindanao del Sur Province on the southern island of Mindanao, the Philippine authorities said on Friday. It was on its way from Cebu Province to Cotabato City, the capital of a Muslim autonomous region on Mindanao.

The incident occurred during a “routine mission in support of U.S.-Philippine security cooperation activities,” U.S. Indo-Pacific Command said in a statement on Friday.

“The aircraft was providing intelligence, surveillance and reconnaissance support at the request of our Philippine allies,” the statement said. The names of those killed were being withheld until family members could be notified.

The Philippines is the oldest treaty ally of the United States in Asia. U.S. troops have been deployed for years at a military camp in Mindanao to help train Philippine soldiers fighting Muslim separatist militants. In 2017, U.S. Special Forces assisted the Philippine military as it fought Islamic State-linked militants in the southern city of Marawi.