Three U.S. troops and a working dog were killed by a bomb blast on Friday in southern Afghanistan, a U.S. official said, speaking on condition of anonymity.
No information was immediately available about the incident. The NATO-led International Security Assistance Force, or ISAF, in Afghanistan said in a statement three service members died following an improvised explosive device attack.
ISAF did not provide the nationality of the victims.
The Taliban, ousted by U.S.-backed forces in the aftermath of al Qaeda's September 11, 2001 attacks on the United States, is engaged in a summer offensive ahead of the planned withdrawal of most foreign troops by the end of this year.
The United States plans to reduce its force in Afghanistan to less than 10,000 after 2014 and withdraw almost all of them by the end of 2016.