French-led forces in Mali are advancing on the key northern city of Timbuktu, as they press on with their offensive against Islamist rebels. On Saturday Malian and French forces seized Gao, another key northern city. The advance comes as African Union leaders are meeting to discuss sending more troops to Mali. Islamists seized the north of the country last year, but have been losing ground since French forces launched an operation earlier this month. Late on Saturday French Prime Minister Jean-Marc Ayrault said Malian and French troops would arrive "near Timbuktu soon". Overnight they secured Gao - northern Mali's most populous city- after special forces captured the airport and a strategic bridge to the south. Most militants appear to have fled into desert hide-outs and the hunt for them may prove more difficult once all major towns are secure, says the BBC's Thomas Fessy in the capital, Bamako. Troops from Niger and Chad are to assist Malian forces in further securing the town. Also overnight, French forces bombed Islamist position in Kidal, Malian officials say. An army source told AFP news agency that the home of the head of Ansar Dine, the main militant group in northern Mali, had been destroyed in a raid. US refuelling African Union leaders are holding a summit in Addis Ababa in Ethiopia, as members move to deploy troops to help the French-led operation there. Outgoing AU chairman Boni Yayi on Sunday hailed France for its military intervention, saying it was something "we should have done a long time ago to defend a member country". In a statement on Saturday, the AU said it wanted to make "an African Standby Force" operational in Mali soon. African states have pledged nearly 5,700 troops to support French and Malian forces in their campaign. Only a small part of the African force has so far deployed. Meanwhile, the US said it would provide mid-air refuelling for French warplanes. The Pentagon said it had also discussed plans for the US to transport troops to Mali from countries including Chad and Togo. Some 3,700 French troops are engaged in Operation Serval, 2,500 of them on Malian soil. France intervened in its former colony after Islamist launched a push to the south earlier this month. Paris said the whole of Africa, and even Europe, was under threat if the Islamist offensive succeeded. As French and Malian troops moved into Gao, Malian officials spoke of scenes of joy, but also some looting. Malian Foreign Minister Tieman Coulibaly said ahead of the summit in Addis Ababa: "This terrorist group intends to spread its criminal purpose over the whole of Mali, and eventually target other countries." The AU has recommended civilian observers monitor the human rights situation in the areas which have come back under the control of the Malian government. Human rights groups have accused the Malian army of committing serious abuses. Islamist groups and secular Tuareg rebels took advantage of chaos following a military coup to seize northern Mali in April 2012. But the Islamists soon took control of the region's major towns, sidelining the Tuaregs.