Up to 16 Afghans were killed or wounded by a rogue American soldier who walked off his base and opened fire on civilians on Sunday, Afghan and Western officials said.
"Today at around 3:00 am a US soldier walked off his base and started shooting at civilians. Ten to 16 people are killed and injured," Ahmad Jawed Faysal, a spokesman for the governor of southern Kandahar, told AFP.
A soldier has been arrested over the incident, NATO's International Security Assistance Force (ISAF) said in a statement.
"A United States service member was detained today in connection to an incident that resulted in Afghan casualties in Kandahar province," it said, without specifying the number of victims.
The shooting comes after a series of incidents in which Afghan soldiers turned their weapons on American troops, killing six, in the wake of the burning of Korans at a military base.
But there was no word on what might have motivated the soldier's actions.
There were also contradictory claims about the death toll and confusion over whether there was more than one incident.
"What we know at this stage is that there have been casualties in two villages, Alokozai and Garrambai villages (in Panjwayi district)," Faysal told AFP.
"A delegation has been sent to find out how this has happened as well as to determine the dead and injured."
Kandahar is a stronghold of Taliban insurgents fighting to oust the government of President Hamid Karzai, which is supported by some 130,000 US-led NATO troops.
The shooting is likely to increase tensions as Washington and Kabul negotiate a long-term strategic partnership deal governing their relations after US-led combat troops pull out in 2014.
The US embassy said it had received reports of a protest in Kandahar province's Panjwai district and warned American citizens to "exercise caution", noting that previous protests have escalated into attacks on Western targets.
The Koran burning ignited days of violent anti-US demonstrations in which some 40 people died, plunging relations between foreign forces and their Afghan allies to an all-time low and forcing US President Barack Obama to apologise.
Afghan resentment of US forces was also provoked by a video posted online in January showing US Marines urinating on the bloodied corpses of slain Afghan insurgents -- an incident condemned by the Pentagon.
And in March last year, the US military apologised after pictures surfaced of US soldiers from a rogue army unit posing with dead Afghans.
Five soldiers from the unit were charged with murder for allegedly shooting civilians for sport and in November the ringleader of the "kill team" was found guilty and sentenced to life in prison by a military panel.
Of the 60 NATO troops killed so far this year, 18 percent -- almost one in five -- have died at the hands of Afghan colleagues, including four French and an Albanian, as well as the six Americans.
The treaty being negotiated between the US and Afghanistan would likely cover the legal status of US troops remaining in the country after 2014 to help Kabul with intelligence, air power and logistics in the fight against the insurgents.
In Iraq, Washington abandoned its pursuit of a strategic partnership deal and pulled out all its troops, leaving no residual force, after failing to get Baghdad to grant its soldiers legal immunity.