France is ready to stop Islamist militants who control northern Mali if they continue their offensive, French President Francois Hollande has said.
However, Mr Hollande said France would only act under UN authorisation.
He was responding to a plea by Malian President Dioncounda Traore for help to counter a renewed rebel offensive.
Earlier, an emergency meeting of the UN Security Council called for the rapid deployment of an African-led international force to Mali.
The Malian army said on Friday that French troops, as well as soldiers from Nigeria and Senegal, were already in Mali. However, speaking at a press conference, the army official refused to give details of their activities.
Residents of the strategic central town of Mopti told the BBC they had seen French troops helping Malian forces prepare for a counter-offensive against the Islamists in nearby Konna.
France was previously believed to have about 100 elite troops in the region. It also has a military base in Chad.
'France will respond'
Armed groups, some linked to al-Qaeda, took control of northern Mali in April 2012.
They have sought to enforce an extreme interpretation of Islamic law.
Western nations are concerned that Mali's north could become a base for terrorists to plan and launch international attacks.
The UN has approved plans to send some 3,000 African troops to Mali to recapture the north if no political solution can be found. But that deployment was not due to take place until September at the earliest.
Mr Hollande said the rebels were "trying to deliver a fatal blow to the very existence of this country".
"France, like its African partners, cannot accept this. 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."
In the wake of Mr Hollande's speech on Friday, France advised all "non-essential" French citizens to leave the West African country.
EU foreign policy chief Catherine Ashton said the situation called for "enhanced and accelerated international engagement".
She said the EU would support the rapid deployment of an African-led mission and "accelerate preparations for the deployment of a military mission to Mali to provide training and advice to the Malian forces".
In December, the EU approved the deployment of about 200 instructors, who were not expected to arrive before the end of February.
Diplomatic sources said Mr Hollande and Mr Traore would meet for talks in Paris next Wednesday.
'No point rushing in'
The BBC's Hugh Schofield in Paris says it is not clear what form French intervention might take, but one possibility is the use of air strikes if the rebels advance on Mopti.
Speaking shortly before Mr Hollande's address, French War Veterans Minister Kader Arif appeared to rule out a speedy deployment of French troops to Mali.
"There is clearly an emergency but... there's no point in rushing in," said Mr Arif.
"At the same time, there can be no kind of engagement that could take place in this emergency without taking account of the international scale."
Earlier this week, the militant Islamist group Ansar Dine said it had entered Konna and intended to advance further south.
Following its emergency meeting on Mali on Thursday, the UN Security Council called for a "rapid deployment" of the African force and expressed "grave concern" at the capture of Konna by "terrorists and extremist groups".
UN diplomats in New York said President Traore had appealed for help to Paris and to UN chief Ban Ki-moon.
France was the colonial power in Mali until 1960.