Two young fighters of the Islamist group Movement for Oneness and Jihad in West Africa (MUJAO) walk in the streets of Gao
The UN Security Council has called for the "swift deployment" of an international force to Mali.
The call comes after Islamist militants said they had entered the key central town of Konna, advancing further into government-held territory.
The UN has approved plans to send some 3,000 African troops to Mali to recapture the desert north, which is controlled by the militants, reports the BBC.
Mali's president has asked the UN and France for help, diplomats say.
France would respond to the request on Friday, France's ambassador to the UN Gerard Araud said.
For logistical reasons the African force already approved by the UN was not expected to even begin its offensive before September or October, the BBC's Barbara Plett reports from the UN in New York.
The UN resolution also calls for peace talks between the government and local rebels, in an attempt to separate them from the foreign extremists, our correspondent reports.
These were set to begin this month but the renewed fighting threatens their chances of success, she adds.
Some European leaders have voiced concerns that jihadists could use Mali's vast Islamist-controlled area, which is the size of France, to launch attacks on Europe.
The latest fighting is the most serious since the militants captured the north from government forces in April 2012.
A spokesman for the Ansar Dine militant group, Sanda Abu Mohammed, said their fighters had driven out government forces from Konna, about 700km (435 miles) north-east of Bamako.
"We are actually in Konna for the jihad [holy war]," he is quoted by AFP news agency as saying.
The army has not commented on the claim.
Earlier, army sources had said that soldiers had advanced on Douentza, a central town held by another Islamist group.
A resident in Douentza said no fighting had so far taken place for control of the town, about 800km (500 miles) north-east of the capital, Bamako.
It was still in the hands of the Movement for Unity and Jihad in West Africa (Mujao), the resident said.
On Tuesday, African Union chairman Thomas Boni Yayi said NATO should send forces to Mali to fight the Islamists.
He said the Malian conflict was a global crisis which required NATO to intervene, in the way it had done in Afghanistan to fight the Taleban and al-Qaeda.
NATO troops should work alongside an African force in Mali, he said.
Ansar Dine and Mujao have controlled most of northern Mali since last April.
They formed an alliance with Tuareg rebels, over-running government forces in the northern regions of Timbuktu, Kidal and Gao in the chaos following a military coup in March.
But their alliance quickly collapsed, with the Islamists capturing the region's main urban centres and marginalising the Tuareg rebels.
The Islamists have been accused of war crimes and attempting to impose a strict version of Sharia, prompting fears the region could become a regional hub for al-Qaeda-linked militancy.
Burkina Faso is trying to mediate an end to the conflict.