CAIRO — The Arab League asked the United Nations Security Council on Saturday to impose a no-flight zone over Libya in hopes of halting Col. Muammar el-Qaddafi’s attacks on his own people, providing the rebels a tincture of hope even as they were driven back from a long stretch of road and towns they had captured in the three-week war.

The extraordinary move by the 22-nation bloc — an extremely rare invitation for Western military forces on Arab territory — increases the pressure on the Obama administration, which has been reluctant to intervene in a war that could turn out to be prolonged and complex.

However, by inviting the West to take such action, it also clears the way for the United States and Europe to press for a strong Security Council resolution and to counter the objections of China and Russia, which traditionally oppose foreign intervention in a country’s internal disputes.

But the United States has not said whether it would pursue a resolution, and it was far from clear that, even if action were forthcoming, it would be enough to stall the march of Colonel Qaddafi’s troops eastward. As the rebels withdrew from the strategic oil town of Ras Lanuf 100 miles east to Brega, and by nightfall on to Ajdabiya, superior government forces pressed their advantage on an insurgency that began as a disparate protest movement and, despite efforts to build a government and an army, remained chaotic, splintered and largely leaderless.