Washington ? A U.S. Army helicopter crashed at sea in the Philippines Thursday with 12 Americans aboard.

A search by another U.S. helicopter and other American military aircraft found no survivors, but the search was continuing, said Navy Cmdr. Jeff Davis, a Pentagon spokesman.

There were no initial indications that the helicopter was brought down by hostile fire, Davis said.

The CH-47 Chinook helicopter was flying a routine resupply mission from the island of Basilan in the southern Philippines to the tiny islet of Mactan, Davis said. It crashed in a gulf north of Basilan at 1:30 p.m. EST (2:30 a.m. Friday in the Philippines), he said.

The air base at Mactan is a logistics base for the anti-terrorist training operation being conducted by U.S. forces on Basilan.

It was not immediately clear how many aboard the Chinook were crew and how many were passengers, Davis said. Typically that helicopter is operated by a crew of three. He did not know the service branch of the passengers but all were U.S. military; the crew’s Army unit was not immediately known.

The Chinook was flying in tandem with another CH-47 at the time it went down about 30 minutes after taking off from Basilan. The second CH-47 conducted an aerial search and was being joined by other American military aircraft.

Davis identified the crash site as waters about 120 miles north-northeast of the city of Zamboanga, off the coast of the southern island of Mindanao.

About 160 U.S. Army special forces soldiers are in the midst of deploying to Basilan, where the Abu Sayyaf terrorist group is holding missionaries Martin and Gracia Burnham of Wichita, Kan., and Filipino nurse Deborah Yap. As of Wednesday, U.S. officials said 80 U.S. troops already were there and the rest were due in a day or two.

The U.S. forces are training Filipino forces in anti-terrorist tactics and techniques.