Five U.S. military personnel were killed in a helicopter crash in southern Afghanistan, the Pentagon said on Tuesday, although there were no immediate details about the cause of the crash. The NATO-led International Security Assistance Force in Afghanistan said the crash occurred on Monday. It happened on the same day that two U.S. troops and five Afghan police and soldiers were killed by a person in an Afghan military uniform in the restive east of the country. "The cause of the crash is under investigation, however initial reporting indicates there was no enemy activity in the area at the time of the incident," ISAF said in a statement. The Pentagon said all five killed in the crash were Americans. New Defense Secretary Chuck Hagel left Afghanistan earlier on Monday after a difficult first trip to Kabul. His visit was marred by a suicide bomb attack near where he was holding a meeting, and differences with Afghan President Hamid Karzai, who accused the United States of col