"We got some information indicating there was a threat to the embassy and decided to close at 11.40 (0340 GMT) this morning," Kathryn Taylor said on Friday.
She refused to give more details. "I'm afraid I can't be more specific."
Taylor said the embassy would remain closed at least until Tuesday because Monday is a public holiday in the United States. "We'll reassess on Monday whether to open on Tuesday," she said.
The US embassy was closed briefly in September 2004 after a white powder was discovered in a mailed envelope. The powder was later found to be non-toxic.
In October this year, suspicious packages containing a yellow, oily liquid - and in one case a powder - were delivered to about a dozen embassies, including the US, British, French, German, Australian, Canadian and Russian missions.
Analysts said the packages, which included a message condemning the countries for mistreating Muslims, were probably sent by scaremongers with no serious ties to well-known terrorist groups.