A U.S. statement said the fighting began when Taliban fighters attacked a U.S.-Afghan patrol with rockets and gunfire.
The Helmand region is the center of Afghanistan's illegal opium-poppy cultivation.
In eastern Afghanistan, in the province of Paktika, near the border with Pakistan, four Afghan soldiers and one Afghan civilian were killed by a suicide bomber at a joint U.S.-Afghan base.
Officials said the bomber, wearing an Afghan national army uniform, walked up to a security gate for Afghan soldiers at the base on Saturday, and detonated his explosives.
Six Afghans were reported injured in the blast.
(compiled from agency reports)