Four U.S. soldiers were killed and another wounded in combat in eastern Afghanistan, the military said in a statement Thursday.
The casualties occurred Wednesday while coalition forces were trying to block the movement of enemy forces in the eastern Nuristan province.
Reinforcements and air support were called in to continue the assault through the night, but it was unclear if there were any enemy casualties.
Late Tuesday, militants bombed two coalition convoys in southern Afghanistan, killing one civilian bystander and wounding 13, including six Canadian soldiers, the military said.
The attacks came as the U.S.-led coalition warned that major battles were likely to continue as Taliban fighters resist a large-scale military push in southern Afghanistan ahead of an imminent security handover to NATO-led forces.
"We are seeing the enemy operating in larger groups. They are fighting hard. They are clearly trying to stop our efforts to move into certain areas," coalition spokesman Col. Tom Collins said.
Coalition and Afghan forces launched Operation Mountain Thrust in earnest last week with more than 10,000 Afghan, British, Canadian and American troops deploying in the largest anti-Taliban offensive since the former regime's 2001 ouster.
More than 600 people, mostly militants, have been killed since May. At least 10 coalition soldiers also have been killed in combat in that time.
The Associated Press contributed to this report.