A suicide bomber blew himself up at a checkpoint in eastern Afghanistan on Wednesday, killing eighteen Afghan nationals and three American service members, U.S. and Afghan officials said. More than 30 others were injured.

The attack happened at around 11:30 a.m. local time when an insurgent walked up to a checkpoint in the Spin Jumat area of Khost City and detonated his explosives vest. A joint Afghan-NATO convoy was in the vicinity of the checkpoint, but it was not clear if it was the target of the attack.

Provincial officials said seventeen Afghan civilians were killed and 32 others were injured. The NATO-led International Security Assistance Force (ISAF) also confirmed three of its service members were killed, who were identified as Americans by a U.S. official. Also killed was an Afghan interpreter.

The U.S. Embassy in Kabul strongly condemned the attack, describing it as 'cowardly.' "Such disregard for the lives of civilians and soldiers further demonstrates that the Taliban and other insurgents indiscriminately continue their murderous campaign against all, including women and children," it said in a statement.

No group immediately claimed responsibility for the attack.