Nato is investigating a report that an Afghan soldier shot and killed foreign troops in the south of the country.
The report, from an Afghan news agency, quoted the Taliban saying three members of the Nato-led Isaf force had been killed in an attack in Helmand.
A Nato spokesman said they were "aware of the incident in Helmand" and were investigating.
In July a renegade Afghan soldier shot and killed three British army Gurkhas at a base in Helmand province.
The report on Saturday by the Pakistan-based Afghan Islamic Press agency (AIP) quoted a Taliban spokesman saying the Afghan soldier had opened fire inside a military base in Sangin district and had then fled to join Taliban fighters.
Correspondents say AIP has strong links to the Taliban and is not generally regarded as reliable.
However, Helmand's deputy police chief told AFP news agency a soldier had attacked Isaf troops.
"We know that an army soldier opened fire on Isaf forces, but we don't know the details," Col Kamaludin Khan said.
The US and Nato have more than 150,000 troops in Afghanistan fighting an insurgency which is concentrated in the southern provinces of Helmand and Kandahar.
Nato's International Security Assistance Force is training and mentoring Afghanistan's security forces, but there have been several incidents of Afghan soldiers firing on foreign troops.
In October, an Afghan soldier fired a rocket-propelled grenade at French and Afghan forces manning an outpost north of the capital, Kabul, but caused no casualties.
In July, as well as the attack on British troops, an Afghan soldier killed two US contractors inside a military base in the north.
In August, two Spanish police officers and an interpreter were killed by an Afghan policeman they were training in north-west Afghanistan.
Spanish PM visits
Spanish Prime Minister Jose Luis Rodriguez Zapatero, in an unannounced visit to Afghanistan on Saturday, said Spanish troops were not there to stay but were committed to ensuring security for Afghans.
He visited Spanish troops serving with Nato-led international forces, accompanied by his Defence Minister Carme Chacon and Foreign Minister Trinidad Jimenez.
"We are not here to stay but we have a firm commitment until the Afghans can guarantee their own security," he told Spanish radio. "When we achieve this they will have won a better future and we all will have gained a more secure future."
It is his second visit to Afghanistan since he took office in 2004.