The United States is temporarily withdrawing more staff from its embassy in Libya's capital for security reasons, but hopes to send them back early next week, the US State Department has said.

"This is a temporary further drawdown of staff for security reasons. We will review our posture again early next week with the goal of restoring staff as soon as conditions allow," a State Department official said in New York on Thursday, where Secretary of State Hillary Clinton is attending the U.N. General Assembly.

US Ambassador to Libya Christopher Stevens and three other Americans were killed during what Washington has called a terrorist attack on the US Consulate in Benghazi on September 11.

The senior official declined to say how many staff were being withdrawn or discuss specifics, but a statement on the website of the US embassy in Tripoli warned of possible demonstrations in Tripoli and Benghazi on Friday.

"The demonstrations are a continuation of those that took place in Benghazi on September 21-22, 2012," it said, referring to protests in support of democracy and against the Islamist militias that Washington blames for the assault on its consulate. The demonstrations erupted into violence.

"Even demonstrations that are meant to be peaceful can become violent and unpredictable. You should avoid them if at all possible," the embassy said.

A source with access to official US threat assessments said no specific threat or specific target had been identified.

However, the source said the staff withdrawal was ordered out of an abundance of caution because some believe the anti-American protests in Benghazi were used as cover by the militants who carried out the September 11 attacks on the US Consulate.