ZAMBOANGA, Philippines ? A U.S. Army helicopter carrying 10 Americans crashed into the sea early today after ferrying troops in a counter-terrorism exercise with the Philippine military, U.S and Philippine officials said.

No survivors were found in hours of searching after the crash, lowering hopes of retrieving anyone alive from the water. Both U.S. and Philippine military forces were searching the southern Philippines waters.

The helicopter wasn’t brought down in an attack, officials from both countries said. “There was no hostile ground fire,” Philippine military spokesman Lt. Col. Danilo Servando said.

The U.S. Pacific Command said the MH-47E Chinook was carrying eight crew members and two passengers. Officials said earlier that 12 Americans were aboard.

Some debris and an oil slick were spotted at the crash site five miles from tiny Apo, a tiny island in the Bohol Sea off the larger island of Negros. Coast Guard Lt. Armand Balilo said searchers found one of the helicopter’s rotors.

Apo was transformed into a staging area for the search-and-rescue effort. Some U.S. and Philippine forces involved in the joint exercise were being pulled away to help.

“You can understand naturally that our hearts are full of sorrow right now,” Maj. Cynthia Teramae, a U.S. spokeswoman for the exercise, said in Zamboanga. “What we are doing is focusing all our attention on the family members.”

Police said rain was falling and visibility was poor at the time of the crash, but the weather had improved several hours later.

Officials said the helicopter had just made three night flights between Zamboanga, a major Philippine military base, and nearby Basilan island, parts of which are a stronghold of the Muslim extremist group Abu Sayyaf. The rebels are holding a missionary couple from Kansas, Martin and Gracia Burnham of Wichita, and a Filipino nurse.

Along with a second MH-47E, the ill-fated helicopter left Zamboanga shortly after midnight for a two-hour flight to Mactan, an islet near the city of Cebu where the United States has a supply base for the Basilan mission, Servando said. It crashed around 2:30 a.m. (1:30 p.m. EST Thursday) in deep water.

The second MH-47E conducted an aerial search and was joined by a U.S. Navy P-3 and a U.S. Air Force C-130 airplane, as well as Philippine aircraft and ships.

Col. Alexander Aleo, commander of the Philippine military’s 103rd brigade headquarters on Basilan, said “everything looked normal” as the choppers dropped off the last of 160 U.S. special forces and supplies that have been arriving over the last two weeks.

The Americans have been operating under extremely tight security, with transport planes landing at night with all of their lights off. Aleo said the two helicopters used a system in which one landed  never turning off its engines  while the other hovered above as a defensive measure.

“There was no mechanical trouble reported or any rebel activity that might have affected their flight,” Aleo said.

Aleo said the American troops monitored developments by radio before deploying this morning to outlying military camps from the main military base on Basilan.

The U.S. soldiers who arrived overnight were packing bags, cleaning their guns or trying to catch a quick nap.