US officials have held face-to-face talks with representatives of Col Muammar Gaddafi's government, the US state department has confirmed.
The US said the meeting reiterated its demand that Col Gaddafi step down, and involved no negotiations.
A spokesman said the Libyan government supported dialogue with the US but only if it was free from preconditions.
On Monday, rebels said they had pushed government troops westwards after seizing back most of the town of Brega.
The Libyan government denied the claim, insisting that the key oil refinery town was still firmly under its control.
'Delivered a message'
In a statement, the US state department said officials had given representatives of Col Gaddafi a "clear and firm" message that the Libyan leader had to go.
"The message was simple and unambiguous - Gaddafi must leave power so that a new political process can begin that reflects the will and aspirations of the Libyan people," it said.
The US did not give the location of the talks, but Libya said they had taken place on Saturday in neighbouring Tunisia.
Libyan government spokesman Moussa Ibrahim hailed the talks as an important step in "repairing relations" with the US.
"We support any dialogue, any peace initiative as long as they don't decide Libya's future from without," he told journalists in Tripoli.
"We will discuss everything but do not condition your peace talks. Let the Libyans decide their future," he added.
Washington said that Jeffrey Feltman, the US assistant secretary of state for near eastern affairs, and the US ambassador to Libya Gene Cretz, had been involved in the talks, but would not say who represented the Libyan side.
American officials said no further talks were planned "because the message has been delivered".
The BBC's Christian Fraser says he has been told that in recent weeks the French have held similar meetings with Libyan officials in the Tunisian resort of Djerba - the difference between the two approaches, he says, is that the French have specified that Col Gaddafi must be sent into exile.
So far, the leader has laughed off suggestions he would leave the country.
US forces have been involved in military action in Libya since the UN in March passed resolution 1973, which permitted intervention to protect Libyan civilians against Col Gaddafi.
Following the resolution, the US took the lead in the air strikes against Col Gaddafi's forces.
Nato later took over - although the US continues to play a support role.
Nato strikes
Meanwhile, fighting continues in Brega, where rebels have been trying to push back pro-Gaddafi forces since Thursday, often fighting at close range in residential areas.
Brega, about 750km (465 miles) east of the capital Tripoli, has changed hands several times in the fighting along Libya's Mediterranean coast since the rebellion began in February.
Rebels said they had pushed government forces back to Ras Lanuf - and they say intercepted radio chatter suggests the forces were led in retreat by their commander, Col Gaddafi's son Mutassim. The government has dismissed the claims outright.
The government says 500 rebels have died in the fighting for Brega, while the rebels say the figure is 12.
Correspondents say the fall of Brega would be a major breakthrough for anti-Gaddafi forces. For weeks the Libyan conflict has appeared to be in a protracted stalemate with rebels holding eastern Libya and pockets in the west.
Nato aircraft have been targeting pro-Gaddafi forces near Brega in recent days, reporting hits on armoured vehicles and rocket launchers near the town, according to Reuters.
On Monday, South Africa's President Jacob Zuma, who has led a mediation mission on behalf of the African Union, said that Libya needed a democratic government.
But he said that the Libyan people must decide their own destiny, and that if Col Gaddafi goes conditions must be in place as to when, where and how that happens.
Mr Zuma made his comments during a joint news conference in South Africa's capital, Pretoria, with the visiting British Prime Minister David Cameron.
Meanwhile, Russia has refused to recognise the rebel leadership as the legitimate Libyan government, with Foreign Minister Sergei Lavrov saying such a move amounted to taking sides in a civil war.
