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 in the town told BBC they heard gunfire on the outskirts, believed to be the Malian army firing warning shots.The rebels said on their website they meant to dislodge the Malian political and military presence in the town.Two other important centres, Kidal and Gao, fell to them in recent days.There has been no real fighting as yet, witnesses said.In their statement, the National Movement for the Liberation of Azawad (MNLA) said they had "liberated” the town of Gao on Saturday.Timbuktu is the only major northern town that remains under the control of the Malian army.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.