The U.S. embassy in Stockholm was evacuated on Wednesday over a possible security incident, a spokesman for the mission told AFP, while police said a suspect envelope had sparked the scare.

"We've evacuated the embassy while we are investigating a possible security incident," embassy spokesman Jeff Anderson said.

"We are working together with the Swedish authorities."

When contacted by AFP around midday, Anderson said the evacuation had taken place "a few hours ago."

Swedish police said on their website that a patrol had been dispatched to the embassy after "a letter with unidentified content had arrived at the embassy".

News agency TT reported the letter contained an unidentified white powder, though police would not confirm that.

"It's a content one would not expect to receive," Stockholm police spokesman Albin Neverbery said.

"When it comes to a sensitive location like an embassy, we take no chances," he said, adding: "A police bomb squad has taken care of the envelope. We're securing the site."

Police, who had no information about any threats issued against the embassy, were dispatched to the scene at 11:35 a.m. and were still there in the early afternoon.

Both embassy staff and visitors were evacuated from the building during the scare. The Stockholm mission has some 170 employees. (AFP)