Libyan rebels looked to push deeper into government-held territory east of the capital on Friday after dismissing an offer from a son of Muammar Gaddafi to hold elections.
Saif al-Islam told an Italian newspaper on Thursday the elections could be held within three months and his father would step aside if he lost, but that proposal was swiftly rejected by the rebel leadership and the United States.
Rebel advances towards Tripoli have been slow, while weeks of NATO strikes pounding Gaddafi's compound and other targets have failed to end his 41-year-old rule.
The rebellion rose up four months ago to the day in the eastern city of Benghazi and NATO intervention in Libya has now been going on for nearly 13 weeks -- longer than many of its backers anticipated -- and the strains are beginning to show within the alliance.
French Armed Forces spokesman Thierry Burkhard suggested the rebels were beginning to hone in on Gaddafi's stronghold of Tripoli.
"The opposition forces seem to have taken the ascendancy on Gaddafi's troops, which shows just how much attrition they are
enduring," he told reporters on Thursday.
The rebel advance, he said, was "essentially in the West and in a belt they are now developing around the Tripoli region."
A Reuters team in Dafniya, the outskirts of the rebels' western bastion of Misrata, described rebels firing artillery and rocket launchers on Friday with a range of about 20 kilometres (miles). Rebels said they were aiming for tanks and munitions in Naimah near Zlitan.
Zlitan, just 160 km (100 miles) from Tripoli, is the next major town on the Mediterranean coast road to the capital. Capturing it would be a major victory.
The rebels have said they would not attack Zlitan because of tribal sensitivities but have been recruiting fighters from the town and waiting for the local inhabitants to rise against Gaddafi.
Rebel forces are fighting Gaddafi's troops on two other fronts: in the east of the country around the oil town of Brega and in the Western Mountains southwest of Tripoli.
They have made slow but important gains in the past few weeks in the mountains and near Misrata, bringing the front closer to Tripoli from the east and southwest.
Rebels said late Thursday an attack on their positions near the town of Ajdabiyah wounded at least 16 fighters in what may have been a friendly fire incident caused by a NATO air strike.
A NATO spokesman said the alliance had no information on the incident.
NATO planes resumed bombardments of Tripoli late on Thursday with eight loud explosions ringing out in the south of the city.
A visiting Russian envoy to Tripoli said the Libyan leadership had reiterated that Gaddafi's departure was a "red line" that could not be crossed.
Libyan Prime Minister Al-Baghdadi Ali Al-Mahmoudi appeared to throw Saif al-Islam's potential concession into question, saying the leader of the revolution was not concerned by "any referendum".
Gaddafi has called the rebels "rats" and says NATO's campaign is colonial aggression to steal Libya's oil.
The rebel leadership in Benghazi rejected the election offer.
"We tell him (Saif al-Islam) that the time has passed because our rebels are at the outskirts of Tripoli, and they will join our people and rebels there to uproot the symbol of corruption and tyranny in Libya," rebel spokesman Abdel Hafiz Ghoga told Al Jazeera television.
A US State Department official also dismissed the election idea, saying it was "a little late for that".
The proposal -- which follows a series of moves the Libyan leader's officials portray as concessions but Western powers dismiss as ploys -- comes at a time when frustration is mounting in some NATO states at slow military progress.
NATO officials have said they may not have the resources for a sustained campaign, and Republicans in the U.S. Congress have questioned the legal grounds for continued US involvement and threatened to cut off funding.
Libya-watchers say Gaddafi is seeking to exploit any divisions among his opponents.
Adding to the pressure on NATO, Russia and China issued a declaration underlining their concerns about the air strikes.