KABUL, Afghanistan - An Afghan soldier opened fire on foreign troops yesterday at a military base in western Afghanistan, killing an American soldier and wounding two Italian troops, a senior Afghan commander said.

The incident - unusual but not unprecedented - could heighten an already considerable sense of mistrust between Western troops and Afghan security forces. That in turn could undermine the Obama administration's plan for Afghan forces to eventually shoulder the responsibility for safeguarding the nation and take the place of the more than 110,000 Western troops now serving in Afghanistan. An additional 30,000 American troops are to arrive in the coming year.

Both Afghan and Western officials acknowledge serious gaps in the training and equipping of Afghan soldiers and police, and the poorly paid Afghan forces have also shown themselves vulnerable to Taliban infiltration.

NATO's International Security Assistance Force declined to provide more information about the circumstances of the shooting, which it said was under investigation.

The Afghan National Army's regional commander, Gen. Jalandar Shah Bahnam, said the shooting occurred at a jointly run base in the Bala Murghab district of Baghdis province, which borders Iran and has been the scene of several days of clashes.

Western and Afghan troops have been hunting for three Afghan police officers missing after their post was attacked over the weekend. Dozens of insurgents had also massed and attacked other Afghan security posts, causing the coalition to rush in Western reinforcements.

Bahnam said the shooting followed an altercation between the foreign forces and their Afghan counterparts that broke out when Western troops refused to let an Afghan soldier approach an area where a helicopter was about to land. After the Afghan soldier opened fire, Western and Afghan troops fired back, wounding him.

Adding to the confusion, a Taliban spokesman said that insurgents had shot down a helicopter in Baghdis yesterday. It was not immediately known if that claim was related to the shooting incident.

The episode comes amid a new outbreak of tensions between Western commanders and the government of President Hamid Karzai over the issue of civilian casualties. Karzai yesterday dispatched a team to investigate reports by provincial officials and tribal elders that 10 Afghan civilians had been killed in a strike by coalition forces in Kunar province, in the remote northeast.

Western military officials said those killed during fighting Sunday were all insurgents, and put the toll at nine.

Meanwhile, Afghanistan's main intelligence agency disclosed that four Taliban fighters, including a local commander, were arrested earlier this month in connection with the Sept. 2 assassination of Abdullah Laghmani, deputy chief of Afghanistan's National Directorate for Security. He died in a suicide bombing outside a crowded mosque with 22 other people.