A Pentagon-contracted Beechcraft King Air 300 crashed in the municipality of Ampatuan on Thursday, killing all four aboard, including a U.S. service member. The plane was en route from Cebu Province to Cotabato City when the accident occurred during a “routine mission in support of U.S.-Philippine security cooperation activities,” according to a statement from U.S. Indo-Pacific Command on Friday.

The aircraft was engaged in intelligence, surveillance, and reconnaissance to support Philippine allies. The names of the deceased are being kept private until families are notified. The Philippines has a long-standing military relationship with the U.S., dating back to being the oldest treaty ally in Asia. U.S. troops have been stationed in Mindanao for years to assist local forces in combating Muslim separatist organizations. Notably, in 2017, U.S. Special Forces helped the Philippine military during the battle against Islamic State-connected militants in Marawi.