American troops have shot dead four Afghan soldiers outside the US embassy in Kabul.
The US embassy is opposite an Afghan garrison
Senior military sources told the BBC that there had been an exchange of fire involving Afghan soldiers and the embassy guards.
A senior Afghan official described the incident as a misunderstanding.
The US embassy blamed heightened tension for the incident, and regretted the loss of life.
The British embassy in the Afghan capital has advised its citizens to maintain a low profile and avoid public places.
The BBC's Kylie Morris in Kabul said the shooting took place just after 1000 local time (0630GMT), but there are varying accounts of what actually happened.
International security forces say there was an exchange of fire after Afghan forces stationed in the garrison opposite the embassy shot at a passing car.
The guards at the US embassy gates are then reported to have returned fire, killing four soldiers and wounding several others.
But according to an official of the Afghan interior ministry, the Americans fired when they grew suspicious of Afghan soldiers transporting weapons to a nearby depot.
Kabul police chief Basir Salangi said a US embassy official had described the incident as a "misunderstanding".
"American soldiers guarding the entrance to the US embassy opened fire by mistake on a group of Afghan soldiers who were unloading some material from the other side of the road, in front of an Afghan security agency building," Mr Salangi told French news agency AFP.
The Americans thought it was an attack attempt, it was a misunderstanding
Kabul police chief
"The Americans thought it was an attack attempt, it was a misunderstanding."
According to AFP, French peacekeeping troops from the International Security Assistance Force (ISAF) said they were caught in the crossfire.
Major Jackie Fouquereau, spokesman for the French contingent responsible for security in Kabul, said a French army vehicle was passing the embassy at the time.
"They had to stop the vehicle and take cover," he said.
"Our soldiers shot twice in the direction of an Afghan soldier who was shooting in the direction of the embassy.
"We took him prisoner and also took under charge another injured Afghan."
There has been no official comment from the Americans.
Our correspondent said the US embassy must qualify as Kabul's most heavily fortified building, guarded by watchtowers and protected by sandbags.
Armed Afghan soldiers permanently guard three national military buildings near the embassy.
She said that after the shooting, tensions on the streets were high as local intelligence officers and soldiers moved to disperse the crowd that had gathered to find out what had happened.
Afghan President Hamid Karzai has been informed and is reportedly planning a formal investigation of the incident.
It comes as the US is on high alert for
terrorist attacks and follows suicide bombings in Morocco and Saudi Arabia.