* France announces military intervention in Mali
* Foreign minister confirms air strike took place
* France says rebels aimed to take over whole country (Adds detail, quotes)
By John Irish and Catherine Bremer
PARIS, Jan 11 (Reuters) - France said it launched a military operation in Mali on Friday to help the government there stem a push southwards by Islamist rebels who control much of the north, in a turnaround from its earlier stance against intervention by its forces.
President Francois Hollande said a surge by the rebels into new territory this week had heightened the security situation and prompted France, backed by the U.N. Security Council and West African nations, to respond to a plea for help from its former colony.
Foreign Minister Laurent Fabius said he would not reveal strategic details of the intervention - such as whether French troops were on the ground - while it was ongoing so as to limit the rebels' knowledge of the operation but he confirmed that French forces carried out an air strike.
"The situation had deteriorated seriously in the last few days. The rebels decided to move south with the apparent goal of taking over all of Mali to install a terrorist state," Fabius told a news conference.
"We must stop the rebels' offensive otherwise the whole of Mali will fall into their hands, creating a threat for Africa and even for Europe."
He said that as far as he knew France was the only country assisting the Malian government militarily for now but the deployment of African troops to join the intervention would be speeded up.
France's intervention, carried out within a day of an appeal for help from Mali's interim president Dioncounda Traore, marked a turnaround for Hollande.
He had earlier backed a plan to send African troops into Mali but hoped to avoid spearheading military action due to concern for several French hostages being held in the region by Islamist rebels.
Fabius said the government had warned the hostages' families that the rebel advance meant France could not hold off any longer.
He urged the roughly 6,000 French expatriates in Mali to leave the country if possible, but diplomatic sources said no evacuations were planned for the time being.
Fabius also said that measures had been taken to safeguard the Malian capital Bamako.
Hollande said backing from the Security Council, which met on Thursday over the Malian request, provided the legal framework for France to intervene in the crisis and said he was supported by West African nations.
Western powers fear that the al Qaeda-linked militants, who seized the northern two-thirds of Mali in April and pushed south to seize the central town of Konna this week, will seek to use the vast desert zone as a springboard for international attacks.
"The terrorists should know that France will always be there when the rights of a people, those of Mali who want to live freely and in a democracy, are at issue," Hollande said as he announced the military operation to reporters in Paris.
"This operation will last as long as is necessary," he said.
Diplomatic sources said French forces would focus on halting the rebels' advance and would then pause to decide their next steps.
Fabius said French Defence Minister Jean-Yves Le Drian would brief reporters further on the military operation on Saturday.
The Defence Ministry in Mali said Nigeria and Senegal were also providing assistance against the Islamist insurgents but Fabius said he could not confirm that. (Additional reporting by Julien Ponthus, Elizabeth Pineau and Vicky Buffery; Writing by Catherine Bremer; Editing by Alison Williams)