France will support Mali's request for military assistance to help counter an offensive by Islamist rebels, Francois Hollande has said.
Mr Hollande said they would help but strictly within the framework of a UN security council resolution.
"They are trying to deliver a fatal blow to the very existence of this country," he said. "France, like its African partners and the whole of the international community, cannot accept this."
"We are faced with a blatant aggression that is threatening Mali's very existence. I have decided that France will respond, alongside our African partners, to the request from the Malian authorities. We will do it strictly within the framework of the United Nations Security Council resolution. We will be ready to stop the terrorists' offensive if it continues."
Mali asked for military help from France after residents of the strategic northern town of Konna said Islamist rebels drove out the Malian army on Thursday, the fiercest fighting since militants took control of the country's north nine months ago.
The fall of Konna was a major setback to government forces, who said earlier on Thursday they were making headway against the alliance of al Qaeda-linked rebels.
The UN Security Council convened emergency consultations in New York and agreed on a statement in which the members "express their grave concern over the reported military movements and attacks by terrorist and extremist groups in the north of Mali, in particular their capture of the city of Konna.
"This serious deterioration of the situation threatens even more the stability and integrity of Mali, and constitutes a direct threat to international peace and security," the council said after the meeting, which was requested by France.
It also repeated calls for restoration of democracy in Mali and urged UN members "to provide assistance to the Malian Defence and Security Forces in order to reduce the threat posed by terrorist organizations and associated groups."
US Ambassador Susan Rice described the request for help from Mali, which was sent to the Security Council: "It wasn't specific, but it basically said, 'Help, France.'"
Western and regional governments are keen to dislodge the Islamists from a desert zone of northern Mali larger than France, which they captured in April, amid concerns they may use it as a launch pad to stage attacks.
Konna was the last buffer between the rebels and Mopti, about 50 km (30 miles) south, which is the main town in the region and is seen as the gateway to the country's north.
After hours of gun battles, heavily armed Islamist fighters paraded in triumph through Konna's centre, saying they would push on to take Mopti and its neighbouring town of Sevare, residents said.
"We took the barracks and we control all of the town of Konna," MUJWA rebel group spokesman Oumar Ould Hamaha told Reuters. "The soldiers fled, abandoning their heavy weapons and armoured vehicles."
News of the fall of Konna sowed panic in Mopti and Sevare, the latter the site of a large military barracks and airport. The towns lie at the crossroads between Mali's desert north and the greener, more populous south.
"We have received the order to evacuate," said the local head of a US aid agency. "We have already pulled all our personnel and material out of Mopti."
While a UN-sanctioned intervention by African troops is unlikely before September, due to logistical constraints, world powers could decide to act sooner, a UN diplomat said.
"If the offensive continues, I think there will be an emergency decision by the international community," UN Special Envoy to the Sahel, Romano Prodi, said during a visit to Bamako on Thursday, without elaborating.
Former colonial power France has been among the most outspoken advocates of an African-led military intervention. Many in Mali's military have also been keen to launch a campaign to reverse their rout by the militants in April.
The UN Security Council has approved in principle the idea of an international military intervention in the north, though it has urged African nations to step up detailed planning in consultation with the United Nations.
Source: agencies
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