The leaders of Britain and France visited two of Libya's newly freed cities Thursday, offering broad support for the country's new rulers as provisional authority forces attempted their first significant assault on Moammar Gadhafi's hometown.

British Prime Minister David Cameron and his French counterpart, Nicolas Sarkozy, were welcomed by jubilant crowds in the capital, Tripoli, and the eastern city of Benghazi, cradle of the uprising.

The two men said they would introduce a United Nations Security Council draft resolution Friday that would establish a U.N. mission in Libya, unfreeze Libyan assets, remove the no-fly zone for civilian flights and abolish the international arms embargo.

They said they would assist Libya's new government to extend its authority throughout the country and track down Mr. Gadhafi and members of his inner circle. Mr. Cameron and Mr. Sarkozy also promised that the NATO military campaign would continue for as long as it is needed.

Later Thursday, in a surprise advance, anti-Gadhafi forces broke through loyalist defenses and entered the coastal city of Sirte, 450 kilometers east of Tripoli.

Several thousand battle-hardened fighters from the city of Misrata advanced into the center of Mr. Gadhafi's hometown, where they faced heavy resistance from elite loyalist troops and snipers.

Sirte is one of Mr. Gadhafi's last remaining bastions, along with the desert town of Bani Walid and the southern outpost of Sabha.

In Tripoli, Mr. Cameron called on the fugitive leader and his followers to “give up.” Mr. Sarkozy added that he would urge authorities in neighboring Niger, a former French colony, to detain Mr. Gadhafi's associates who entered the country in recent days.

The visiting leaders held talks in Tripoli with National Transitional Council Chairman Mustafa Abdel Jalil, who said the NTC will reward key allies for their help in ending Mr. Gadhafi's rule as it decides future oil and gas contracts.

Mr. Sarkozy said France is not looking for special treatment for its actions.

Britain and France pushed for NATO action to protect civilians against forces loyal to Mr. Gadhafi, and the French were the first to recognize the NTC as the official representative of the Libyan people.

Turkish Prime Minister Recep Tayyip Erdogan is also scheduled to visit Libya this week as part of a North African tour.