Nato says two of its troops have been shot dead on a base in Afghanistan, the latest of several attacks after the burning of the Koran by US soldiers.
Nato said a man in Afghan army uniform and another in civilian clothes opened fire in southern Kandahar province. The dead are believed to be US soldiers.
Some local officials say there was only one attacker, a teacher at the base.
Hours earlier Nato's top general in Afghanistan said the recent violence was a "setback" that would be overcome.
Nato says shots were fired indiscriminately, claiming two of its soldiers' lives. It has yet to give their nationalities, but US and Afghan officials say the dead were Americans.
Nato believes one of the killers was an Afghan soldier. If so, this would be the third time in a week that a member of the Afghan security forces has killed Nato troops.
What happened is still unclear.
An Afghan army spokesman in Kandahar said two gunmen initially opened fire on an Afghan sentry manning a security tower at the base in Zheray district, before climbing it and shooting at Nato troops. He said both attackers were killed.
But officials in Zheray district dispute this account and say the attack was carried out by only one man - who apparently lived on the base to teach Afghan soldiers literacy.
The local district governor said the man grabbed an Afghan soldier's gun and opened fire. In return, Nato killed both Afghans.
The BBC's Bilal Sarwary in Kabul says it is unclear how a teacher was allowed to stay on the base and have access to a weapon.
The six Nato personnel are amongst more than 30 people killed since protests erupted last week over the burning of the Koran by US troops at another military base.
Troubled flared after US personnel apparently inadvertently put copies of the Koran, which reports say had been confiscated from terror suspects, into a rubbish incinerator at Bagram air base, near Kabul.
Muslims consider the Koran the literal word of God and treat each copy with the utmost respect.
Responding to the attacks, Nato's commander in Afghanistan, Gen John Allen, said: "We have to understand the significance of the great faith of Islam to these people and we have to account for that.
"But it doesn't push the relationship back."
He emphasised that over the 10 years of Nato's presence in Afghanistan, "thousands and thousands" of troops had shown "reverence" for the Islamic faith.
He also said he would be willing to walk, unarmed, into the Afghan Interior Ministry in Kabul, where two senior Nato military advisers were shot dead on Saturday.
Afghan authorities are hunting a 25-year-old Afghan policeman over that shooting.
Last Thursday, two US soldiers were shot and killed by a man wearing Afghan army uniform during protests at a Nato base in eastern Nangarhar province.
A senior Afghan general told the BBC last week: "The virus of Taliban infiltration and rogue soldiers has spread like a cancer. Curing it has not helped. You need an operation."
More than 70 Nato troops have been killed by Afghan colleagues in recent years.
The BBC's Orla Guerin in Kabul says that privately, some Nato officials wonder who they can trust.
Thursday's deaths in Kandahar came as Nato let a small number of its advisers return to their duties at Afghan ministries, the Associated Press reports.