Thousands of African troops are expected in Mali in "coming days and weeks" to support Malian and French forces, France's UN ambassador says.
Gerard Araud said France wanted the deployment of a West African force to happen "as quickly as possible".
France says its air strikes have forced back Islamists who took control of northern Mali last year, though the rebels seized one town on Monday.
France intervened on Friday after the Islamists began advancing southwards.
French officials have said they feared that the Islamists could have marched on the capital, Bamako, creating a grave security threat for the wider region.
Aid workers said many people had been fleeing areas targeted by French air strikes over the past four days.
Mr Araud was speaking after the UN Security Council convened in New York at France's request.
"The African forces are going to be deployed in the coming days and weeks," he said, adding that the Nigerian commander of the force was already on the ground.
The force would be deployed under UN Security Council resolution 2085, which was passed in December and allowed for a 3,000-strong African-led mission to intervene in Mali later this year in the absence of any negotiated solution.
French Foreign Minister Laurant Fabius said the West African force would include 600 troops from Nigeria, 500 each from Niger, Burkina Faso, Togo and Senegal, and 300 from Benin.
He said France's involvement would last "a matter of weeks".
France has sent about 550 troops to the central town of Mopti and to Bamako, and a defence ministry official told Reuters troop numbers would increase to 2,500 in coming days.
At least 11 Malian soldiers and a French helicopter pilot have died in Mali. More than 100 militants are reported to have been killed.
Mr Araud said the Malian army had suffered "heavy casualties", but added that "they are fighting - they are fighting in very difficult circumstances."
A spokesman for the International Committee of the Red Cross said the humanitarian situation was "fast deteriorating"
"Mass displacement of the population has already been observed, casualties have been reported and we're trying our best to address the humanitarian needs of the population," said Ali Naraghi.
On Monday, French officials said Islamists had counter-attacked and taken Diabaly, a town in government controlled territory 400km (250 miles) from Bamako, but denied that it represented a major setback.
"We knew that there would be a counter-attack in the west because that is where the most determined, the most organised and fanatical elements are," French Defence Minister Jean-Yves Le Drian told France's BFM TV.
But Mr Le Drian insisted that France's campaign was "developing favourably".
France intensified its air strikes on rebel targets over the weekend, with its aircraft also bombing the town of Gao in eastern Mali. On Monday witnesses told AFP there had been air strikes on Douentza for a fourth consecutive day.
Residents in several northern towns told AFP news agency that Islamists in several key northern towns including Gao and Douentza had either fled or taken cover from the air strikes by Monday.
Rebels of the al-Qaeda-linked Movement for Oneness and Jihad in West Africa (Mujao), said that France would pay for its intervention.
Meanwhile, Algeria, which has allowed French jets to cross its airspace, said it had closed its long desert border with Mali.
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.
One Islamist group, Ansar Dine, began pushing further south last week, seizing Konna.
The town has since been recaptured by Malian troops with French aerial support.