The U.S. military says two Army helicopter pilots have been killed in an accident on a base in northern Iraq.

The military said in a statement that there were no "enemy forces" present and no hostile fire reported.

It said the aircraft made a "hard landing" Sunday near an airfield inside the base, but gave no further details. The military often uses the phrase "hard landing" to mean a crash.

The statement said the accident was under investigation, and the pilots' names are being withheld pending notification of the next of kin.

The deaths raise to at least 4,378 the number of U.S. military personnel who have died in Iraq since the war began in March 2003. That's according to an Associated Press count.