U.N. Chief Ban Ki-moon said he spoke to Libyan officials and told them government forces must stop attacking civilians.
Ban said Moammar Gadhafi's government had agreed to another visit by a special envoy.
Meantime, Rebels in the port city of Misrata said they took over the local airport Wednesday from Gadhafi's retreating forces, and seized weapons and ammunition, a potentially major breakthrough in efforts to break a two-month siege.
Since the uprising against Gadhafi began in mid-February, Misrata has endured some of the country's most violent battles. Regime troops surrounded the coastal city from three sides and subjected it to daily shelling.
Meanwhile, new NATO airstrikes hit Tripoli on Tuesday. Government administrative buildings appeared to have been the target.
NATO said Tuesday it had conducted more than 6,000 sorties since it assumed command of the aerial operation over Libya on March 31. More than 2,400 of those were airstrikes, a statement said.
The Associated Press contributed to this report.