Libya conflict: NTC forces attack Sirte and Bani Walid
Columns of anti-Gaddafi forces have renewed assaults on Sirte and Bani Walid, two of the last strongholds of the ousted Libyan leader.
Fighters say they have entered northern parts of Bani Walid but have since been met with fierce resistance.
Trucks and tanks are also approaching Sirte, hours after an advance was forced back by Gaddafi loyalists.
Meanwhile the UN General Assembly has given Libya's seat to the ruling National Transitional Council (NTC).
Earlier, Turkish leader Recep Tayyip Erdogan met the country's new rulers in Tripoli, as he continues a regional tour asserting Ankara's regional influence.
His visit comes a day after British Prime Minister David Cameron and French President Nicolas Sarkozy became the first Western leaders to visit since Col Muammar Gaddafi was ousted.
Mustafa Abdul Jalil, leader of the interim authority, is to meet US President Barack Obama during the UN General Assembly meeting in New York on Tuesday, US officials said.
The BBC's Ian Pannell, in Tripoli, says the latest attacks suggest the battle for the remaining contested areas of Libya could be entering a decisive phase.
But concern is growing for the tens of thousands of civilians still believed to be living in both cities, and who have been surviving for weeks with limited access to food, water and electricity, our correspondent adds.
Fighters loyal to the NTC claim to have gone into northern areas of Bani Walid.
Reuters reports that streets in the northern outskirts of the town were deserted and houses were riddled with bullet holes.
Earlier, an unnamed NTC commander told the BBC they had met some resistance but that his forces were now advancing "to the heart of Bani Walid".
"As we were about to advance, a clash took place with a surveillance crew made up of two or three enemy vehicles. They shot at us but thank God, we were able to stop them and defeat them."
Last week the anti-Gaddafi forces said they were hours from taking Bani Walid, which has been under siege for several weeks.
But they were driven back after encountering fierce resistance from Gaddafi loyalists.
The BBC's Peter Biles, outside Bani Walid, says soldiers at a checkpoint told him that the driver of Col Gaddafi's son Saif al-Islam has been captured.
Our correspondent says smoke can be seen and explosions heard from the town. A number of ambulances have been coming from Bani Walid carrying wounded.
One NTC fighter described the centre of Bali Walid as a "ghost town", adding that 500 families were seen leaving in the last two days.
The fighters are facing resistance in a handful of pro-Gaddafi bastions, including the southern outpost of Sabha and Sirte, Col Gaddafi's birthplace.
Scores of trucks mounted with machine guns and four tanks were seen on the road leading into Sirte on Friday, Reuters reported. Anti-Gaddafi sources said that Sirte airport had been taken from loyalists.
On Thursday evening, fighters breached defences south and west of the city, about 8km from the centre, but met heavy resistance, the NTC said.
An NTC spokesman told the BBC that fighters had been forced to retreat about 2km to treat casualties following their incursion into the outlying areas of the city, where they reportedly clashed with snipers in a high-rise tower and an elite unit of pro-Gaddafi troops.
The anti-Gaddafi fighters advancing on Sirte - made up of battalions from Misrata, 200km to the west - suffered at least four deaths and seven wounded, although a report quoting the Misrata Military Council said 11 were killed and 34 hurt.
The UN General Assembly has voted in favour of giving Libya's seat to the NTC, despite some opposition from Latin American governments.
The 193-member assembly voted 114 to 17, with 15 abstentions. Some African nations called for a decision to be postponed.
The move allows Libyan interim PM Jalil to attend a UN General Assembly meeting in New York next week.
US officials said President Obama would meet with Mr Jalil on the sidelines of the gathering on Tuesday to discuss the NTC's plans for the post-Gaddafi era.
The meeting would allow Mr Obama to "congratulate chairman Jalil on the success of the Libyan people of ending the Gaddafi regime", US deputy security adviser Ben Rhodes told AFP news agency.
Nato raids have continued over Libya, striking against pro-Gaddafi military targets.
Britain's Ministry of Defence said aircraft had attacked sites in and around Sabha on Wednesday.
The ministry said targets included a military vehicle depot and a group of buildings which Nato surveillance had confirmed were used by Gaddafi loyalists.
And on Thursday the RAF destroyed a tank, four rocket launchers and four armed vehicles around Sirte, the ministry said.
Meanwhile, on Friday, the NTC is to send a delegation to neighbouring Niger in an effort to recover gold and cash believed to have been taken out of Libya by fleeing Gaddafi loyalists.
At least 36 members of the fugitive leader's inner circle, including relatives and generals, have fled to Algeria and Niger since Tripoli fell to NTC forces last month.
Mr Abdul Jalil said Libya would also ask for the handover of individuals wanted by the International Criminal Court (ICC). The ICC has indicted Col Gaddafi, his son Saif al-Islam and his intelligence chief for crimes against humanity.