Western governments, particularly former colonial power France, had voiced alarm after the al Qaeda-linked rebel alliance captured the town of Konna on Thursday, a gateway towards the capital, Bamako, 600 km (375 miles) south.
President Francois Hollande said France would not stand by to watch the rebels push southward. Paris has repeatedly warned that the Islamists' seizure of the country's north in April gave them a base to attack neighbouring African countries and Europe.
"We are faced with blatant aggression that is threatening Mali's very existence. France cannot accept this," Hollande, who recently pledged Paris would not meddle in African affairs, said in a New Year speech to diplomats and journalists.
The president said resolutions by the U.N. Security Council, which in December sanctioned an African-led military intervention in Mali, meant France was acting in accordance with international law.
French military operations in support of the Malian army against Islamist rebels "will last as long as necessary," France's U.N. ambassador, Gerard Araud, wrote in a letter to the Security Council obtained by Reuters.
In Washington, a U.S. official told Reuters the Pentagon was weighing options in Mali, including intelligence-sharing with France and logistics support.
French Foreign Minister Laurent Fabius confirmed France had carried out air strikes against the rebels to prevent them conquering the whole of Mali. He refused to reveal further details, such as whether French troops were on the ground.
France's intervention immediately tipped the military balance of power, with Malian government forces quickly sweeping back into Konna, according to local residents.
"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.
EU SPEEDS UP DEPLOYMENT
A military operation had not been expected until September due to the difficulties of training Malian troops, funding the African force and deploying during the midyear rainy season. But Mali's government appealed for urgent military aid from France on Thursday after Islamist fighters took Konna.
EU foreign policy chief Catherine Ashton called on Friday for "accelerated international engagement" and said the bloc would speed up plans to deploy 200 troops to train Malian forces, initially expected in late February.
Blaise Compaore, president of neighbouring Burkina Faso, which is acting as a mediator in the Malian crisis, said his country would contribute a contingent of ground troops toward the African Union mission to retake Mali's north.
Burkina Faso had been due to host peace talks between the Malian government and some of the rebel factions on Thursday, but those have been postponed until January 21 due to the outbreak of hostilities.
The capture of Konna by the rebels - who have imposed strict Sharia Islamic law in northern Mali - had caused panic among residents in the towns of Mopti and Sevare, 60 km (40 miles) to the south. Calm returned, however, after residents reported Western soldiers and foreign military aircraft arriving late on Thursday at Sevare's airport - the main one in the region.
Military analysts said the Western soldiers may have been the first deployment of French special forces.
They voiced doubt, however, whether Friday's action heralded the start of the final operation to retake northern Mali - a harsh, sparsely populated terrain the size of France - as neither the equipment nor ground troops were ready.
"We're not yet at the big intervention," said Mark Schroeder, director for Sub-Saharan Africa analysis for the global risk and security consultancy Stratfor. He said France had been forced to act when the Islamists bore down on Sevare, a vital launching point for future military operations.
"The French realised this was a red line that they could not permit to be crossed," he said.
STATE OF EMERGENCY
More than two decades of peaceful elections had earned Mali a reputation as a bulwark of democracy in a part of Africa better known for turmoil - an image that unravelled in a matter of weeks after a military 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.
Interim President Dioncounda Traore, under pressure for bolder action from Mali's military, declared a state of emergency on Friday. Traore will fly to Paris for talks with Hollande on Wednesday.
"Every Malian must henceforth consider themselves a soldier," Traore said on state TV, calling on mining and telecoms companies to contribute to the war effort. He said he requested French air support with the blessing of West African allies.
The chief of operations for Mali's Defence Ministry said Nigeria and Senegal were among the other countries providing military support on the ground. Fabius said those countries had not taken part in the French operation.
A spokesman for the Nigerian air force said planes had been deployed to Mali for a reconnaissance mission, not for combat.
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 eight nationals in Islamist hands in the Sahara after a string of kidnappings.
A spokesman for al Qaeda's north African arm, AQIM, urged France, in a video posted on the Internet, to reconsider its intervention. "Stop your assault against us or you are digging your own sons' graves," said Abdallah Al-Chinguetti.