"French forces brought their support this afternoon to Malian army units to fight against terrorist elements," Hollande told reporters. "This operation will last as long as is necessary."
Hollande said United Nations Security Council resolutions meant France was acting in accordance with international laws.
Earlier, Hollande had made it clear that France would intervene to stop any further drive southward by Islamist rebels as Malian soldiers launched a counter-offensive to wrest back a town captured by militants this week.
Western powers fear the alliance of al Qaeda-linked militants that seized the northern two-thirds of Mali in April will seek to use the vast desert zone as a launchpad for international attacks.
Mali's government appealed for urgent military aid from France on Thursday after Islamist fighters encroached further south, seizing the town of Konna in the centre of the country. The rebel advance caused panic among residents in the nearby towns of Mopti and Sevare, home to a military base and airport.
"We are faced with blatant aggression that is threatening Mali's very existence. France cannot accept this," Hollande said in a New Year speech to diplomats and journalists. "We will be ready to stop the terrorists' offensive if it continues."
The U.N. Security Council in December authorised the deployment of an African-led force supported by European states.
"The French believe that France, and Europe, face a real security threat from what is happening in the Sahel," said Jakkie Cilliers, executive director of the Institute for Security Studies in South Africa.
More than two decades worth of peaceful elections had earned the Mali a reputation as a bulwark of democracy in a part of Africa better known for turmoil - an image that unraveled in a matter of weeks after a coup last March that paved the way for the Islamist rebellion.
Mali is Africa's third largest gold producer and a major cotton grower, and home to the fabled northern desert city of Timbuktu - an ancient trading hub and UNESCO World Heritage site that hosted annual music festivals before the rebellion.
Residents had seen Western soldiers arriving late on Thursday at an airport at Sevare, 60 km (40 miles) south of Konna.
Sevare residents also reported the arrival of military helicopters and army reinforcements, which took part in the counter-attack to retake Konna overnight on Thursday in a bid to roll back the militant's southward drive.
"Helicopters have bombarded rebel positions. The operation will continue," a senior military source in Bamako said.
A source at Sevare airport also said around a dozen war planes had arrived on Friday. A spokesman for the Nigerian air force said planes had been deployed to Mali for a reconnaissance mission, not for combat.
A spokesman one of the main groups in the Islamist rebel alliance said they remained in control of Konna.
Asked whether the rebels intended to press ahead to capture Sevare and Mopti, the Ansar Dine spokesman, Sanda Ould Boumama, said: "We will make that clear in the coming days." He said any intervention by France would be evidence of an anti-Islam bias.
The French foreign ministry stepped up its security alert on Mali and parts of neighbouring Mauritania and Niger on Friday, extending its red alert - the highest level - to include Bamako. France has 8 nationals in Islamist hands in the Sahara after a string of kidnappings.
"Due to the serious deterioration in the security situation in Mali, the threat of attack or abduction is growing," the ministry said in its travel alert. "It is strongly recommended that people avoid unnecessarily exposing themselves to risks."