Two U.S. servicemen were killed Tuesday in southern Philippines as the Hummer jeep they were aboard was hit in a landmine explosion, a Filipino official said.

One Filipino marine soldier was also killed and two others were wounded in the blast near a marine detachment in Indanan town of Sulu province on Tuesday morning, chief military spokesman Lt. Col. Romeo Brawner said.

Brawner initially said one was killed and four others were injured, but he updated the death toll in a text message sent to reporters.

Rebecca Thompson, the spokeswoman of the U.S. Embassy in Manila, said they are investigating the incident but gives no more details

A few hundred of U.S. troops have been stationed in southern Philippines to help their Filipino counterparts fight al Qaeda- linked militants. It remains unclear who planted the landmine.