French and Malian troops are restoring government control over the Saharan trading town of Timbuktu, the latest gain in a fast-moving French-led offensive against al-Qaeda-allied fighters occupying northern Mali.

The Islamist militant rebels have pulled back northwards to avoid relentless French air strikes that have destroyed their bases, vehicles and weapons.

This has allowed French and Malian troops to advance rapidly with air support and armoured vehicles.

A Malian military source said the French and Malian forces reached "the gates of Timbuktu" without meeting resistance from the Islamist insurgents who had held the town since last year.

The French and Malians controlled the airport and were working on securing the town, a UNESCO World Heritage site and labyrinth of ancient mosques and monuments and mud-brick homes, ready to flush out any Islamist fighters still hiding.

"Timbuktu is delicate, you can't just go in like that," the source, who asked not to be named, said.

Earlier the French-Malian offensive recaptured Gao which, along with Timbuktu, was one of three major northern towns occupied last year by Tuareg and Islamist rebels who included fighters from al-Qaeda's North Africa wing, AQIM.

The Malian military source, and at least one resident of Gao who travelled south out of the city, said there were still rebel "pockets of resistance" there, and that government troops were carrying out house-to-house searches.

The third town, Kidal, in Mali's rugged and remote northeast, remains in rebel hands.

The US and Europe are backing the UN-mandated Mali operation as a counter-strike against the threat of radical Islamist jihadists using the west African state's inhospitable Sahara desert as a launch pad for international attacks.

One Timbuktu resident now outside the town said a friend inside had sent him text messages saying he had seen government troops on the streets, but gave no more details.

Fighters from the Islamist alliance in north Mali, which groups AQIM with Malian Islamist group Ansar Dine and AQIM splinter MUJWA, had destroyed ancient shrines sacred to moderate Sufi Moslems in Timbuktu, provoking international outrage.

They had also imposed severe sharia, Islamic law, including amputations for thieves and stoning of adulterers.

As the French and Malian troops push into northern Mali, African troops from a continental intervention force expected to number 7,700 are being flown into the country, despite delays due to logistical problems and the lack of airlift capacity.

France sent warplanes and 2,500 troops to Mali after its government appealed to Paris for help when Islamist rebels early in January launched an offensive south towards the capital Bamako.

They seized several towns, since retaken by the French.

In the face of the two-week-old French-Malian counter offensive, the rebels seemed to be pulling back north into the trackless desert wastes and mountain fastnesses of the Sahara.

Military experts fear they could carry on a gruelling hit-and-run guerrilla war against the government from there.

A leader of Mali's main Tuareg insurgent movement, MNLA, whose initial separatist rebellion in the north was hijacked by al-Qaeda and its local Malian allies, offered help from his group's desert fighters to the French-led offensive.

Speaking from Ouagadougou in Burkina Faso, Ibrahim Ag Mohamed Assaleh said the MNLA was preparing to attack the withdrawing al-Qaeda-allied Islamist forces and its leaders, who he said were hiding in the Tidmane and Tigharghar mountains in the Kidal region that borders with Algeria.