© UnknownAfghan National Army Uniform
Kabul, Afghanistan - A man in an Afghan army uniform opened fire on a group of Americans at a base in the south of the country, killing a U.S. soldier and wounding another, an Afghan military spokesman said Monday.

Spokesman Gen. Mohammad Zahir Azimi said the gunman was also killed in the shootout on Sunday. "Right now, an investigation is going on to determine whether he really was a soldier or someone using an army uniform. And if he was a soldier, what caused the shooting," Azimi said.

Similar attacks have raised fears of increased Taliban infiltration of the Afghan police and army as NATO speeds up the training of the security forces. In some cases the attackers were Afghan soldiers who turned on NATO troops. Others involved insurgents dressed in Afghan uniforms.

A NATO statement released late Sunday said only that a coalition service member was killed in the incident, apparently by an Afghan soldier, but provided no details on the location or the victim's nationality.

Azimi said the shooting occurred inside one of the Afghan National Army's bases in Zabul province, about 250 miles (400 kilometers) south of Kabul.

The shooting brought to 11 the number of NATO soldiers killed this month.

On Dec. 29, an Afghan soldier shot and killed two members of France's elite 2nd Foreign Parachute Regiment, a part of the Foreign Legion. A week earlier, an Afghan soldier opened fire on coalition troops inside an outpost in western Herat province, wounding a number of alliance troops.

NATO is encouraging the rapidly expanding Afghan security forces to take more responsibility ahead of the coalition's 2014 pullout target date. NATO's training mission hopes to have about 350,000 Afghan troops and police trained and ready by then.

Meanwhile, Taliban fighters ambushed a convoy carrying supplies for NATO in western Afghanistan, killing two private security guards and wounding three. Seven of the attacking insurgents were also killed, said deputy police chief Mohammad Ghaws Milyar.

The convoy was heading toward the city of Farah, about 420 miles (700 kilometers) west of Kabul, when it was hit.