Tuareg separatist rebels in Mali say they have surrounded the historic town of Timbuktu after a rapid advance through the north of the country. Eyewitnesses say they have heard heavy weapons and machine-gun fire, reportedly directed at a military base. Troops appear to have deserted the base, a resident told BBC News. On Sunday, the leader of the coup that overthrew the president last month pledged to restore the constitution and return power to civilian hands. Capt Amadou Sanogo said in a statement in the capital, Bamako, that the 1992 constitution, which was scrapped by the coup leadership, would be re-established. Analysts say the rebels in the north have been taking advantage of uncertainty after the coup to make a swift advance. Two other important northern centres, Kidal and Gao, fell to the Tuareg fighters and their Islamist allies in recent days. A resident who spoke to the BBC's Thomas Fessy said the Malian army had fled but a Malian Arab self-defence militia, backed by local businesses, was still defending Timbuktu. There was no actual fighting inside the town, the resident added. But reports speak of residents barricading themselves into their homes with their families. Timbuktu, about 1,000km (600 miles) from the capital Bamako, is the only major northern town still under the control of the Malian army. Figures on casualties in the fighting are not been available. 'Prison opened' In a statement, the National Movement for the Liberation of Azawad (MNLA) said they had "liberated" the town of Gao on Saturday. Few details of the situation in the town of 90,000 people have emerged since the rebels moved in, but witnesses quoted by AFP news agency said unknown attackers had forced open the gates of the prison and several public building had been looted by civilians. Officials with several international non-governmental organisations based in Gao have fled the town, a security source told the agency. The country has been in turmoil for more than a week since army officers overthrew the government, blaming it for failing to contain the rebels, who launched their offensive in January. Capt Sanogo said earlier that soldiers had decided not to fight the rebels in Gao because the town's military camps were close to residential areas. In his brief, new statement on Sunday, he said state institutions would also be "restored". Mali's overthrown President Amadou Toumani Toure is said to be currently safe at an undisclosed location inside Mali. Regional group Ecowas has demanded the reinstatement of Mali's elected government, and has put 2,000 troops on standby for a possible intervention. Appealing for support from the international community, the current chairman of Ecowas, Ivory Coast President Alassane Ouattara, said on Saturday that Mali's territorial integrity must be preserved "at all costs". "We must succeed because if Mali is divided, carved up, it is a bad example," he added. Ecowas has threatened to close land borders, freeze assets and impose a financial blockade if the army does not stand down before Monday. Looted weapons Residents of Bamako fear real shortages if the sanctions take effect, our correspondent says. The fighting in the north has forced some 200,000 people from their homes, with neighbouring states struggling to look after refugees. Tuaregs have launched several rebellions over the years, complaining that the government in Bamako ignores them. However, the new conflict has been fuelled by the return of Tuareg fighters from Libya last year after fighting for the late Muammar Gaddafi or his opponents. It appears these fighters are heavily armed with looted weapons. The MNLA are backed, for now, by a smaller, Islamist group called the Ansar Edine, which wants to impose Sharia law.