President Francois Hollande says French troops are taking part in operations against Islamists in northern Mali.
French troops “have brought support this afternoon to Malian units to fight against terrorist elements”, he said.
Armed groups, some linked to al-Qaeda, took control of northern Mali in April.
Mr Hollande said the intervention complied with international law, and had been agreed with Malian President Dioncounda Traore. A state of emergency has been declared across the country.
The militants said on Thursday that they had advanced further into government-controlled territory, taking the strategic central town of Konna.
Mr Hollande said French military action had been decided on Friday morning and would last “as long as necessary”.
“Mali is facing an assault by terrorist elements coming from the north whose brutality and fanaticism is know across the world,” the French president said.
He said Mali’s existence as a state was under threat, and referred to the need to protect its own population and 6,000 French citizens living there.
French Foreign Minister Laurent Fabius said the aim of the operation was to stop Islamist militants advancing any further.
“We need to stop the terrorists’ breakthrough, otherwise the whole of Mali will fall into their hands threatening all of Africa, and even Europe,” he told reporters.
He confirmed that France had carried out an air strike.
At least seven French hostages are currently being held in the region, and Mr Fabius said France would “do everything” to save them.
British Foreign Secretary William Hague said on Twitter that the UK supported the French decision to help Mali’s government against northern rebels.
Mr Hollande spoke shortly after the Malian army said that French troops, as well as soldiers from Nigeria and Senegal, were already in Mali.
No details of their numbers or activities were given.
The UN had previously approved plans to send some 3,000 African troops to Mali to recapture the north if no political solution could be found, but that intervention was not expected to happen until September.
France ruled Mali as a colony until 1960.