Afghan officials say U.S.-led coalition forces have killed nine Afghan policemen in a friendly-fire incident in western Afghanistan.

Officials say foreign troops entered a district of Farah province Sunday without informing local authorities, who then mistook the troops for Taliban fighters.

Both sides fired on each other for several hours. The U.S.-led coalition says its troops retaliated against what they saw as a hostile force and called in air strikes. It says the incident is under investigation.

In other developments, NATO says one of its soldiers was killed Sunday in a battle in the eastern province of Khost. The soldier's nationality was not disclosed.

Elsewhere, NATO says its troops accidentally killed at least four Afghan civilians Saturday night in the eastern province of Paktika. It says the troops fired two mortars that landed a kilometer short of a militant target, killing the civilians.

NATO says it deeply regrets the accident.

The chairman of the U.S. Joint Chiefs of Staff, Admiral Mike Mullen, said on U.S. television Sunday that there is "mixed progress" in Afghanistan.

He said extremist and terrorist groups are joining forces in northern Pakistan and infiltrating across the Afghan border. But, he denied that the United States and its NATO allies are losing the fight in Afghanistan against the Taliban and other militants.

Some information for this report was provided by AFP, AP and Reuters.