Mali's army has driven back Islamist rebels from a strategic central town after France intervened with air strikes to stop militants controlling the country's desert north from advancing.
One French pilot has been killed in the military action and hundreds of French soldiers have been deployed in the West African country.
French President Francois Hollande took action at the request of interim President Dioncounda Traore, who has declared a state of emergency.
Western governments expressed alarm on Thursday after an al Qaeda-linked rebel alliance captured the town of Konna, a gateway towards the capital Bamako 600km (375 miles) south.
The Malian army has said it was attacking the "last pockets of resistance" by insurgents in Konna after they recaptured it with the help of the French.
It said as many as 100 Islamist fighters had been killed in the battle for Konna.
The UN's Sahel envoy Romano Prodi, left, and President Dioncounda Traore Mr Hollande said the "terrorist groups, drug traffickers and extremists" in northern Mali "show a brutality that threatens us all." He vowed that the operation would last "as long as necessary".
The West African regional bloc ECOWAS has now authorised the immediate deployment of troops to Mali.
The bloc's commission president Kadre Desire Ouedraogo said it made the decision "in light of the urgency of the situation".
For the past nine months, the Islamic militants have controlled a large swathe of northern Mali, a lawless desert region where kidnapping has flourished.
Mr Hollande said the operation was aimed in part at protecting 6,000 French citizens in Mali, including seven who are being held captive.
UN Secretary-General Ban Ki-moon said Senegal and Nigeria also responded to an appeal from Mali's president for help to counter the militants.
Late last year, the 15 nations in West Africa, including Mali, agreed on a proposal for the military to take back the north, and sought backing from the UN.
The Security Council authorised the intervention but imposed certain conditions, including the training of Mali's military, which has been accused of serious human rights abuses since a military coup last year sent the nation into disarray.
Al Qaeda's affiliate in Africa has been a shadowy presence for years in the forests and deserts of poverty-stricken Mali.
Most Malians adhere to a moderate form of Islam, but in recent months the terrorist group and its allies have taken advantage of political instability, taking territory they use to stock weapons and train forces.
The Islamists have insisted they want to impose Sharia only in northern Mali, though there long have been fears they could push further south.
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