World leaders vowed to stand by Libya and help the rebel National Transitional Council (NTC) rebuild the country as they welcomed Libya's new leaders to the international community at United Nations headquarters in New York.

UN secretary-general Ban Ki-moon told the NTC the Security Council had acted to protect the Libyan people from violence.

"Today, we must once again respond with such speed and decisive action, this time to consolidate peace and democracy," Mr Ban said.

Libya's new flag flew at the UN for the first time, with the nation reverting to the national flag that was used from 1951 until Gaddafi, who ruled for nearly 42 years, introduced a green flag for his Libyan Arab Jamahiriya, or "People's Republic", in 1977.

NTC president Mustafa Abdul Jalil promised a spirit of tolerance and reconciliation and appealed for international assistance to help his country emerge from conflict and build democracy.

Foreign Minister Kevin Rudd joined world leaders in voicing support for Libya's new leaders.

"We in Australia are proud of the fact we remain the third largest donor in the world to Libya's ongoing humanitarian needs," he said.

US president Barack Obama urged forces loyal to Moamar Gaddafi to end their fight and announced the return of the US envoy to Tripoli.

"Today, the Libyan people are writing a new chapter in the life of their nation," Mr Obama said.

"We will stand with you in your struggle to realize the peace and prosperity that freedom can bring. Those still holding out must understand - the old regime is over, and it is time to lay down your arms and join the new Libya."

He further pledged: "So long as the Libyan people are being threatened, the NATO-led mission to protect them will continue."

Libya's new rulers, finally recognised on Tuesday by the African Union, are still trying to dislodge well-armed Gaddafi loyalists from several towns and have yet to start a countdown toward writing a new constitution and holding elections.

But former Libyan president Gaddafi has also vowed to fight on, rallying his supporters and saying his regime lives on.

Gaddafi, who is on the run, taunted the Western alliance in a speech broadcast by a Syrian-based television station on Tuesday, saying: "The bombs of NATO planes will not last."