"During dinner, the police commander and his colleagues shot them and then fled. The commander was Afghan National Police in charge of local police in Sangin," a senior Afghan official told the Reuters news agency.
"It looks like he had drawn up a plan to kill them previously," the official said.
Another district official however suggested that the Afghan police had not fired the fatal shots, but had tipped off nearby Taliban fighters who had arrived to kill the Americans.
A spokeswoman for American forces in Afghanistan said it was too early to say if the killer was a rogue member of the Afghan forces, or an infiltrator in disguise.
She said: "Three US Forces-Afghanistan service members died following an attack by an individual wearing an Afghan uniform in southwest Afghanistan today.
"It is USFOR-A policy to defer casualty identification procedures to the US Department of Defence." Such killings have climbed sharply in the past year and have significantly undermined trust between the Nato-led coalition and their Afghan allies.
Before the latest attack, there had been at least 24 such attacks since January, in which 28 people have been killed.
Sangin has been one of the most violent areas of the whole country and was responsible for around a third of British casualties before it was handed over to United States Marines in 2010.
