Like the U.S., Britain has suggested it could also supply weapons to rebel forces in some circumstances - despite an arms embargo covering Libya - but says it has not yet decided to do so.
Rebels in Libya control much of the strategic oil town of Brega, but they're coming under fire from Moammar Gadhafi's forces.
Brega's been the site of battles during weeks of back-and-forth fighting.
The rebels, backed by airstrikes, have been making advances. They have seized much of Libya's eastern coast, but have been unable to push westward toward the capital, Tripoli.
NATO said U.S. warplanes will leave the skies over Libya on Monday. Other allies will provide the air power to conduct missions while the U.S. provides support such as refueling.
Meantime, a Gadhafi envoy is making a diplomatic push in Europe to end the fighting.
On Monday, Italy recognized opponents of Gadhafi as the country's only legitimate voice. Italy is only the third country, after France and Qatar, to recognize the rebel-led Libyan National Transitional Council as North Africa nation's only legitimate governing body.
After speaking with the council's foreign envoy, Ali al-Essawi, Italian Foreign Minister Franco Frattini announced the decision and said the only way to resolve the conflict in the former Italian colony is for Gadhafi to leave.
The Associated Press contributed to this report.