President Barack Obama's deputy spokesman Josh Earnest said that top officials had seen details of the latest attack, which came after NATO troops resumed activities following a review of operations with Afghan forces triggered by a spate of so called "green-on-blue" incidents.

"Make no mistake... these attacks do no diminish in any way the commitment of the president, the commitment of our men and women in uniform or the commitment of our allies to follow through and complete successfully the mission to end the war in Afghanistan in 2014," he added.

Earnest said that the United States and its allies had taken a number of steps to mitigate the risk of insider attacks, including greater vetting of Afghan forces.

His comments came just hours after a firefight between NATO troops and their Afghan allies killed five people in murky circumstances.

A NATO soldier, a civilian contractor and three Afghan troops died in an exchange of fire Saturday evening in Wardak province west of the capital Kabul, the transatlantic alliance's International Security Assistance Force said.

"What was initially reported to have been a suspected insider attack is now understood to possibly have involved insurgent fire," deputy ISAF commander Lieutenant General Adrian Bradshaw told a hastily-called press conference.

The incident remains under investigation.

Provincial police spokesman Abdul Wali told AFP that the shooting broke out after "a verbal dispute" between the two sides.

Ministry of Defense deputy spokesman General Daulat Waziri said the incident was a result of a "misunderstanding", adding that it was investigating whether insurgent fire was involved.

Defence ministers from NATO's 28 member and 22 partner countries meeting in Brussels next week will discuss the alliance's role after 2014, when the Kabul government takes over national security in full, Rasmussen said.

The meeting should set the outline of the programme which will likely focus on training, advice and assistance, with the final mission terms being set before end-2013 so as to ensure a smooth transition, he said.

NATO heads agreed in May at a Chicago summit to launch an assistance mission post-2014 but Rasmussen said he could not give any details as yet.

"Our partners share our interest in cooperative security. They share our commitment to stability and they share the burden of our operations," he said, adding that six countries were ready to contribute to the new mission.

A NATO source named the six as Australia, New Zealand, Finland, Sweden, Ukraine and Georgia.

Over the weekend, Der Spiegel magazine carried a report based on German intelligence sources saying that after Western combat troops leave in 2014, up to 35,000 foreign soldiers would be necessary, mostly to train the Afghan army.