The paratrooper from the 82nd Airborne Division is in stable condition at a field hospital at Bagram Air Base, just north of Kabul, after being shot on a reconnaissance patrol near Lwara in Paktia province.
"We have no information on the people who conducted the ambush," said Roger King, a U.S. military spokesman at Bagram Air Base. "It was apparently a few shots, and then they moved away. They were not engaged."
The injured soldier was initially taken to a special forces base in Orgun-e in neighbouring Paktika province, where he was operated on before being moved to Bagram.
He is likely to be further evacuated to Landstuhl Regional Medical Center in Germany on Thursday.
American soldiers wear bulletproof vests, but the round struck the soldier near the armpit, missing his ballistic protection.
The United States operates a special operations base in Lwara, very close to the Pakistani border. It has come under rocket attack in recent weeks, although no injuries have been reported.
U.S. military officials have reported a pronounced increase in attacks against coalition forces recently, after months of mounting operations to search for al Qaeda and Taliban remnants.
"This could be the closest thing to an offensive they can mount," King said. "We've got repeated small contacts."
Earlier Wednesday, unidentified attackers struck after dawn against an Afghan army base in south Kabul on Wednesday, and 11 guerrillas, a soldier and a civilian were killed in the firefight, authorities reported. It was the most serious such battle in the capital area in months, and followed two days of attacks on U.S. military positions, in which no U.S. personnel were killed.
Police Col. Haji Rashid said Wednesday's attack on the Afghan army base began about 7 a.m. when the guerrillas, armed with AK-47 semi-automatic rifles, rushed the army's Kabul garrison in the Bagram-i District, about six miles south of the center of the capital.
Soldiers fought back mainly with rocket-propelled grenades, he said, and afterward bodies were strewn about the ground in front of the base.
He said 11 of the attackers and an Afghan army soldier were killed. A wounded civilian died en route to a hospital, said Maj. Angela Herbert, a spokeswoman for the International Security Assistance Force, the multinational force that patrols Kabul.
Rashid said a soldier and one of the guerrillas were wounded in the clash, which he said lasted close to an hour. He said 20 soldiers and 20 police were involved.
The police commander said it could not be determined immediately whether the attackers were Afghans, and he did not know how many may have escaped. "We have yet to complete our investigation," he said.
"They came from the south of Kabul, from the direction of the mountains and villages," he said.
Maj. Herbert of the multinational force said its largely European troops were not involved in the action, although its does patrol the area regularly. She said the force sent a quick-reaction patrol to the scene, with an ambulance, and was transporting the wounded civilian to a hospital when he died.
Tuesday, U.S. special forces soldiers killed four attackers.
Roger King, a spokesman at Bagram Air Base, just north of Kabul, said about 30 U.S. special operations soldiers were conducting a vehicle-mounted, reconnaissance patrol near Asadabad, in the eastern province of Konar, when they came across five men in a vehicle.
After initially reporting that all five men fired at the American soldiers, U.S. officials later said the firefight was precipitated when one of the men in the car leveled an AK-47 at the soldiers and pulled the trigger.
The rifle apparently misfired. "Only one person tried to fire his weapon, and it misfired," said Christa D'Andrea, a spokesperson at Bagram.
"Because he showed hostile intent, U.S. forces engaged. No shots were fired from the vehicle, due to the misfire," D'Andrea said.
U.S. officials said that when the rifle was recovered, a bullet was chambered and that the hammer was against the firing pin, suggesting that the gun had indeed misfired.
On Monday, an American patrol was ambushed in the same area outside of Asadabad. In that incident, U.S. forces say they killed two attackers.
The revised details of the incident brought the American version of events closer into line with an account carried by the Pakistan-based Afghan Islamic Press (AIP) news agency on Wednesday, which Kind had denied.
AIP said four Afghan civilians were killed when U.S. troops opened fire on their vehicle after the driver apparently failed to stop when signaled.
King said no U.S. personnel were hurt in the clash. The wounded attacker was taken to a local hospital, he said.
King said U.S. soldiers searched the attackers' vehicle and found a "large amount" of "regional," not Afghan money.