President Francois Hollande Tuesday said France was on guard after a militant Islamist website called for his assassination in revenge for intervening militarily in Mali and the Central African Republic.

"This is not the first time that there have been threats," Hollande said. "We are extremely vigilant."

French prosecutors meanwhile opened a probe following the threats of revenge attacks by the al Minbar Jihadi Network, which is close to al-Qaida, in a series of messages online.

"To our lone wolves in France, assassinate the president of disbelief and criminality, terrify his cursed government, and bomb them and scare them as a support to the vulnerable in the Central African Republic," a message said.

"Neither Hollande, nor his soldiers will know peace in France as long as the Muslims of Mali and the Central African Republic cannot live properly in their country," it said.

A source in the French president's office said that although the government was alert, threats of this nature were not new.

"This is not the first time there have been threats," the source said. "There were others during the Mali intervention and even before, so we took precautionary measures."

"Just because they (threats) are being publicized does not mean that they are new... Sometimes they are more dangerous when they are not publicized."

France sent in troops to Mali in January last year to drive out Islamist militants and Tuareg rebels who had taken control of the sprawling desert north.

Although their onslaught on the south, where the capital Bamako is located, was halted, fighters loyal to Islamist groups still operate there and stage periodic attacks.

France has also sent around 2,000 troops to the Central African Republic in support of a 6,000-strong African Union peacekeeping mission following a wave of Muslim-Christian bloodletting in the aftermath of a coup.