President Barack Obama will make his case for US involvement in Libya to an anxious American public tonight.

White House aides were reluctant to spell out details of Mr Obama's speech in advance. However, deputy national security adviser Denis McDonough said his reasoning could not be applied to escalating clashes between pro- and anti-government forces in Syria and elsewhere.

"Obviously there are certain aspirations that are being voiced by each of these movements, but there's no question that each of them is unique," Mr McDonough said. "We don't get very hung up on this question of precedent."

He sidestepped questions about whether Mr Obama would lay out an exit strategy for US actions in Libya, saying only that Secretary of State Hillary Clinton would discuss those details during an international meeting on Libya in London tomorrow.

Mr Obama's speech comes after the administration scored an important diplomatic victory. Nato ambassadors yesterday approved a plan for the alliance to assume from the US command all aerial operations, including ground attacks.

That will help Mr Obama assure Americans he can deliver on his promise that the United States will be a partner in the military action against Libya, but not from the driver's seat. Bickering among Nato members delayed the process.