Obama signed the order, known as a presidential "finding", within the last two or three weeks, according to government sources familiar with the matter.

Such findings are a principal form of presidential directive used to authorise secret operations by the Central Intelligence Agency. This is a necessary legal step before such action can take place but does not mean that it will.

In addition to the CIA operatives, dozens of British special forces and MI6 intelligence officers are also working in Libya.

News that Obama had given the authorisation surfaced as the President and other US and allied officials spoke openly about the possibility of sending arms supplies to Gaddafi's opponents, who are fighting better-equipped government forces.

The US is part of a coalition, with NATO members and some Arab states, which is conducting air strikes on Libyan government forces under a U.N. mandate aimed at protecting civilians opposing Gaddafi.