Columns of anti-Gaddafi forces have renewed assaults on Sirte and Bani Walid, two of the final strongholds of the ousted Libyan leader. Gunfire and explosions have been heard around Bani Walid's hills and valleys, 180km (110 miles) south of Tripoli, as hundreds of fighters move on the town. Trucks and tanks are also approaching Sirte, hours after an advance was forced back by Gaddafi loyalists. Meanwhile, Turkish PM Recep Tayyip Erdogan is visiting Libya's capital. He has 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 UK PM David Cameron and French President Nicolas Sarkozy became the first foreign leaders to visit since Col Muammar Gaddafi was ousted. Resistance 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 country's interim authorities, the National Transitional Council (NTC), have already seized a valley leading into Bani Walid following their push on Friday, Reuters news agency reported. An unnamed NTC commander told the BBC fighters were making their way into Bani Walid. "We were planning to hold Friday prayers, God willing, in Bani Walid," he said. "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." He said the Gaddafi loyalists had now retreated and his forces were advancing "to the heart of Bani Walid". 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. 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. NTC chairman Mustafa 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.