Diplomats say the decision was due to “security reasons”.
Embassy spokesman Chris Long said that staff were evaluating the situation, and it was not possible to say when the embassy would reopen.
Last month, a suspected Tamil Tiger suicide bomber detonated explosives strapped to her body at a police station right in front of the US embassy, which is located near the British High Commission.
But diplomatic sources say there is no evidence that the latest incident is in any way connected with the Tamil Tigers.
