Malian government forces on Friday launched an offensive against Islamists who control the north of the country with backing from the French and other foreign military, military and political sources said.
The attack was aimed at stemming advances made by Al-Qaeda-linked radicals who this week triggered international alarm with a push south towards the capital, Bamako, a military officer told AFP.
"Our offensive has started," the officer said on condition of anonymity. "The objective is to retake total control of the town of Konna and to proceed from there."
He added that "military planes from friendly countries" were being used in the offensive while a Malian government official said the operation had the support of French and European armed forces.
"European military, including French, are present in Mali to repel any southward advance by the Islamists," the official said, speaking on condition of anonymity.
With the situation on the ground developing rapidly, France advised all its expatriates not involved in essential services to leave Mali immediately.
French Defence Minister Jean-Yves Le Drian was in contact with US Defense Secretary Leon Panetta and was due to talk to his British and German counterparts later in the day.
The offensive came a day after Mali's government appealed to France, the former colonial power, and the United Nations for help in pushing back the Islamists.
As well as capturing Konna, the rebel forces have moved about 1,200 Islamist fighters to within 20 kilometres (12.5 miles) of Mopti, a strategically important town on the frontier between rebel-held and government-held territories.
French President Francois Hollande responded to Mali's appeal on Friday by saying Paris was ready to help, under the auspices of the UN.
The UN Security Council has already given its blessing for a 3,000-strong African force to be sent to Mali but it will not be ready to deploy before September at the earliest.
That has put pressure on France to act quickly and Hollande acknowledged that the situation had become critical.
"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.
"I have decided that France will respond without delay and alongside our partners, to the request of the Malian authorities."
He added: "We will do it strictly in the framework of UN Security Council resolutions and we are ready to stop the terrorist offensive if it continues."
Mali's interim president, Dioncounda Traore, will visit Paris on Wednesday for talks with Hollande.
The Islamists' advance has exacerbated fears of Mali becoming an Afghanistan-style haven for terrorists within easy reach of western Europe.
France has led calls for an international response but had insisted its involvement would be limited to providing logistical support for the force being prepared, slowly, by the West African regional bloc ECOWAS.
On the ground in central Mali, witnesses told AFP that foreign troops and weapons had already begun arriving by transport plane on Thursday, but it was unclear which country they came from.
French sources said Paris has only 14 military liaison officials in Mali, apart from secret service agents.
Diplomats at the UN in New York said overnight that Traore, had appealed for help to Paris and UN chief Ban Ki-moon in a letter US ambassador Susan Rice summed up as: "Help -- France."
Until this week, the Islamists had stayed in Mali's vast arid north, where they took control nine months ago, taking advantage of a power vacuum after a coup in March.
Political tensions in the capital, Bamako, and doubts about the capacity of the Malian army have contributed to the delays in the planned African force being deployed.