The head of Libya's interim authorities said, two sons of Libyan leader Col Muammar Gaddafi who were holed up in the town of Bani Walid until Saturday have now left.Mustafa Abdul Jalil said that Saif al-Islam and Mutassim Gaddafi had been blocking the town's surrender.Earlier, rebels said, negotiations for Bani Walid's surrender had broken down and an assault was imminent. But Mr Abdul Jalil said, the talks would continue until a deadline on Saturday.Anti-Gaddafi forces belonging to the National Transitional Council (NTC) have moved into position near Bani Walid, 150km south-east of the capital Tripoli. Bani Walid is one of four towns and cities still controlled by Gaddafi supporters.Meanwhile Col Gaddafi's chief of security, Mansour Daw, is reported to have crossed into neighbouring Niger. Reports said he had arrived in the northern Niger city of Agadez, along with 10 other people.