The US military has suffered its 2,000th death in the Afghan war - with a suspected "insider" attack at a checkpoint in the east of the country.A US soldier and a foreign contractor were killed while two Afghan soldiers died and four were injured.
The nationality of the contractor was not given immediately.
Attacks by rogue elements in the Afghan security forces sharply increased this year, prompting the coalition to suspend joint operations this month.
Requests for joint operations must now be approved by regional commanders.
The American death toll goes back to the US-led invasion of Afghanistan in 2001.
The new deaths occurred on Saturday in Wardak province, a spokesman for the Nato-led International Security Assistance Force said.
The incident took place at a checkpoint near an Afghan National Army base in the district of Sayedabad, according to Afghan officials
An Afghan soldier turned his gun on Americans and started shooting, Shahidullah Shahid, a provincial government spokesman, told the Associated Press news agency.
"Initial reports indicate that a misunderstanding happened between Afghan army soldiers and American soldiers," he said.
It is unclear what happened to the Afghan soldier who allegedly opened fire first.
Military officials from both sides have launched a joint investigation.