The 34 deaths came amid some of the worst fighting in Afghanistan since the ouster of the Taliban in 2001 and reinforced fears of a resurgence of Islamic extremists.
A 24-hour spasm of violence killed some 120 people earlier this week, before calming briefly. It erupted again Friday with six militants, an Afghan soldier and a civilian killed in Helmand province, the main opium poppy-growing region, where drug profits are believed to fund the insurgency, said Gen. Rehmatullah Raufi, military commander for the south.
Hours later in the same area, insurgents crouching among fields of grapevines and wheat opened fire on a kilometre-long convoy of Afghan army trucks as they snaked their way slowly along a dirt road with reinforcements, he said.
The two sides exchanged fire with machine-guns and AK-47 assault rifles for six hours before the insurgents fled on foot and motorbikes, the general said.
When it was over, 15 rebels and four soldiers lay dead, while 13 troops were still missing, an army officer said on condition of anonymity because he was not authorized to speak to the media.
Hundreds of extra forces were then rushed into the area and it is now controlled by the army, Defence Ministry spokesman Gen. Zahir Azimi said.
Militants ambushed another Afghan army convoy in southern Zabul province Saturday and four rebels were killed as the troops returned fire, Raufi said.
The U.S. soldier was killed Friday in Uruzgan province, also in the south, the military said in a statement. Six soldiers were wounded and in stable condition.
Lieut. Tamara Lawrence, a U.S. military spokeswoman, said the U.S. soldiers were conducting a joint patrol with Afghan forces when they encountered enemy fighters about 10 a.m. Friday.
In the past year, Uruzgan has been the site of some of the heaviest fighting in Afghanistan, but militants suffered high losses in battles with coalition forces, and the violence has subsided in recent months.
At least 235 members of the U.S. military have died in and around Afghanistan as a result of the U.S. invasion of Afghanistan in late 2001, according to the U.S. Defense Department.
Two French special forces troops were killed Saturday while fighting the Taliban in Kandahar province, the French Defence Ministry said. No other details were immediately available.
France has had 200 special forces officers in southeast Afghanistan since 2003 as part of the U.S.-led coalition.
In the western city of Herat, an explosion ripped through a vehicle carrying a former warlord, Amanullah Khan, wounding him and two others, said Gulam Sarwar Haydari, the city's deputy police chief. It was not immediately clear what caused the blast.
Violence has been steadily increasing across Afghanistan for the past 18 months, despite the presence of some 23,000 U.S. troops and 9,000 others mainly belonging to a separate NATO-led peacekeeping force. About 2,300 Canadian troops are involved in the mission.
The militants have changed their tactics, using more suicide and roadside bombings, like those in Iraq, which is proving difficult to counter.