An African Union summit is to discuss the conflict in Mali as African states move to deploy troops to help the French-led operation there.
African states have pledged 7700 troops to support French and Malian forces in their campaign against Islamist militants in northern Mali.
Only a small part of the African force has so far deployed.
French-led troops have retaken several towns since France intervened two weeks ago, and on Saturday advanced on Gao.
The French defence ministry said troops gained control of the city - northern Mali's most populous - after securing the airport and a strategic bridge to the south.
However, French and Malian officials later told the Associated Press news agency that only parts of Gao were under their control, and that the operation there was ongoing.
French officials said troops from neighbouring Niger and Chad would move into Gao to help secure it.
Gao's mayor, who had been ousted when Islamists seized control of northern Mali last year, returned to the town on Saturday.
The other major northern cities, Kidal and Timbuktu, remain in Islamist hands. French forces are now reported to be targeting Timbuktu and Lere, to the west.
The BBC reports France now has 2500 soldiers deployed in Mali since entering the conflict on 11 January to stop the rebels advancing further south.
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.