An explosion killed two soldiers from the Nato-led force and a civilian in Afghanistan's southeastern province of Khost on today, the force said.
Two more soldiers of the International Security Assistance Force (ISAF) were wounded in the explosion which happened during a routine patrol, it said in a statement. No further details were given.
It did not identify the casualties, but American troops form the bulk of the Nato-led force in Khost which lies near the border with Pakistan.
Earlier in the day, two police were killed after their vehicle was hit by a roadside bomb in the province, officials said.
Violence has surged since 2006 in Afghanistan where more than 12,000 people, including some 380 foreign troops, have been killed, the bloodiest period since Taliban's removal in 2001.
The al Qaeda-backed Taliban lead an insurgency against the government and foreign troops that back it and the militants largely rely on suicide raids and roadside bomb blasts as part of their insurgency.