The UN Security Council has called for a swift deployment of foreign troops to Mali to rein in ultra-conservative armed groups in charge of the country's north.
The call on Friday comes as the fighters are vowing to capture more territory in the West African nation.
Diplomats at the UN in New York said Dioncounda Traore, Mali's interim president, had appealed to Paris and UN chief Ban Ki-moon for help.
Citing a letter from the president, Susan Rice, the US ambassador to the UN, said: "It basically said: 'Help - France'."
France is Mali's former colonial ruler and the two countries maintain bilateral relations.
Francois Hollande, the French president, told a meeting of diplomats in Paris on Thursday his country would respond urgently to Mali's appeal, but would only act under the auspices of the UN.
"They are trying to deliver a fatal blow to the very existence of this country," Hollande said of the Islamist groups that control the north of Mali.
"France, like its African partners and the whole of the international community, cannot accept this."
Al Jazeera's Rory Challands, reporting from Paris, said French military officials would neither confirm nor deny reports that French troops had already arrived in Mali.
"We did hear from a spokesman with ECOWAS, the bloc of West African nations, that the UN has endorsed to go in with a military force into Mali … They did almost as good as confirm that the French were there but they're saying the French are there purely to train Malian troops and to offer logistical support."
Following an emergency meeting on Mali, the Security Council called for a "rapid deployment" of an agreed African force to the country and expressed "grave concern" at the capture of the town of Konna by "terrorists and extremists groups".
ECOWAS has agreed to deploy a force of up to 3,000 to help end the insurgency, which gained momentum and saw the capture of large swathes of territory in the wake of a coup staged by junior army officers last March.
The officers behind the coup said it was prompted by the failure of the government to contain the rebellion.
Although UN officials had warned that no troops were likely to arrive before September, witnesses told the AFP news agency that foreign troops and weapons had already begun arriving.
AFP said witnesses said the troops arrived by transport plane on Thursday to bolster government forces in central Mali, but it was unclear what country they came from.
France had so far offered only logistical support to the regional force.
Witnesses told AFP of military aircraft landing with weapons and foreign soldiers at an army base in Sevare, just 60km from Konna.
One witness at the airport reported seeing weapons and soldiers leaving a C-160 military transport aircraft, adding: "Some of the men were white."
A Malian official, confirming the arrival of the military aircraft, said they included one plane from a European nation that left men and equipment at Sevare, but did not say which one.
Earlier on Thursday, Abdou Dardar of Ansar Dine, one of the groups occupying northern Mali, told AFP that Islamist fighters had taken Konna, northeast of the regional capital of Mopti.
"We almost entirely control the town [of Konna]. Afterwards, we are going to continue" pushing south, Dardar said by phone. Witnesses told AFP that Malian troops were retreating.