French troops have begun military operations including air strikes in Mali to contain Islamist groups which are continuing to clash with the army in a fight for control of the desert north of the west African country.

François Hollande announced on Friday night that French armed forces had gone to the aid of Malian troops on the ground during the afternoon. The French president said Mali was facing a "terrorist aggression" of which "the whole world now knows its brutality and fanaticism".

The foreign minister, Laurent Fabius, said France's air force carried out an air strike in Mali on Friday as it supported government forces.

Al-Qaida-linked groups seized the northern two-thirds of Mali last April, a month after a military coup that followed the army's desertion of a military campaign against Tuareg and Islamist rebels. Western powers fear militants could use the vast desert in the former French colony as a launchpad for international attacks.

France said it was acting with the backing of west African states. It had responded to an appeal for military assistance from Mali's embattled interim president, Dioncounda Traoré, after Islamists seized the town of Konna in the centre of the country, about 375 miles north-east of the capital, Bamako, on Thursday.

On Friday night a defence ministry official said the Malian army had retaken control of Konna. "The Malian army has retaken Konna with the help of our military partners. We are there now," Lieutenant Colonel Diaran Kone told Reuters, adding that the army was mopping up Islamist fighters in the surrounding area.

Hollande said recent UN security council resolutions provided the legal framework for him to respond to the request.

The very existence of the "friendly" state of Mali was under threat as well as the security of its population and that of 6,000 French expatriates, Hollande warned.

The military operation would last "as long as necessary", he said. The French parliament will debate the move on Monday.

Hollande added: "Terrorists must know that France will always be there when it's a question, not of its fundamental interests, but of the rights of the Malian population to live freely and in democracy."

Late last year, the 15 countries in west Africa, including Mali, agreed on a proposal for the military to take back the north, and sought backing from the United Nations. The UN security council authorised the intervention but imposed certain conditions. These include training for Mali's military, which has been accused of serious human rights abuses since the coup.

Traoré used a live televised address on Friday night to announce a state of emergency for the next 10 days, and called on mining companies and non-governmental bodies to donate trucks to the military effort.

The announcement fuelled doubts about the capacity of Mali's army, which has been notoriously under-resourced for years. It received a boost in recent weeks, however, by the arrival of equipment that was impounded in nearby Guinea under the terms of an embargo imposed after last year's military coup.

Konna is less than 40 miles from the strategic city and army base of Mopti. Boubakar Hamadoun, editor of the Bamako-based newspaper Mali Demain, who has reporters based in the north, said there were Islamists controlling Konna "but they are integrated into the population". "It is very difficult for the army to fight them," he said. "It is a very complicated situation."

Sources in Mopti reported panic there , with evacuations of women and children, as residents anticipated clashes between Malian and foreign troops and Islamists could reach the town.

Hamadoun cast doubt on reports that Douentza, one of the southernmost towns under Islamist control, had been recaptured by the Malian army this week. "There are some army personnel in Douentza in strategic positions, but the rebels are still very much in control of the town," he said.

The renewed fighting follows the disintegration of a ceasefire between one of the Islamist groups, Ansar Dine, and the government. It has sparked panic in Mopti and other towns south of the de facto border between government and Islamist control, and prompted concerns in the international community that the Islamist groups – which operate a drug trafficking and kidnap economy in northern Mali and other Sahelian countries – could capture more ground.

The security council condemned the capture of Konna and called on UN member states to provide assistance to Mali "in order to reduce the threat posed by terrorist organisations and associated groups."

A regional military intervention approved by the UN had not been expected to start before September. Hollande's announcement marked a radical departure from recent agreements that limited the role of French and other international forces to providing Mali's army with training and logistical support.

France, the former colonial power in Mali and other countries in the Sahel region, has hundreds of troops stationed across west and central Africa. This month it declined to provide military intervention in another former colony, the Central African Republic, whose government is also under threat from rebel groups.

The European Union said it would speed up measures to send 200 trainers to improve the effectiveness of the Malian army. EU foreign policy chief Catherine Ashton said on Friday that recent rebel advances underlined the need for "enhanced and accelerated international engagement" to help restore state authority throughout Mali.

"The European Union … will accelerate preparations for the deployment of a military mission to Mali to provide training and advice to the Malian forces," Ashton said in a statement.

France advised its expats on Friday to leave. The British Foreign Office also advised all Britons to leave Mali by commercial flights as soon as possible.

In a revision of its travel advice because of this week's fighting, it warned against any travel to the country. It is thought fewer than 100 Britons are currently in Mali.

The UK has no troops in the country at the moment, but has committed itself to help the EU military training mission.

The foreign secretary, William Hague, said the UK supported the French intervention.

"UK supports [the] French decision to provide assistance to [the] Government of Mali in the face of [the] rebel advance," Hague said in a message on Twitter. A Foreign Office spokesman said Hague was offering "political support".