Three American service members were killed this morning in a suicide attack carried out by a man on a donkey, according to NATO and Afghan officials.

The attack happened this morning in the Sayedabad district of Wardak province. Afghan officials say a suicide bomber riding a donkey approached a joint patrol of U.S. and Afghan soldiers, and detonated as he approached troops.

NATO isn't confirming the nationalities of the troops, but a senior Afghan official tells ABC News they were American.

A translator, whose nationality is not known, was also killed in the blast. Four Afghan soldiers and a number of civilians were hurt.

Wardak is a restive province in eastern Afghanistan. Earlier this year, Afghan President Hamid Karzai ordered all U.S. Special Forces out of the region, after accusations of improper conduct. Afghan forces are now responsible for all security in the province, but are often accompanied on missions by American troops, whose mandate is to advise and assist when requested.