France has asked the United Nations to "accelerate" implementation of a resolution that enables the deployment of an international force to Mali.

The letter also formally notified the Security Council that it had responded to a request for help from Mali's interim president Dioncounda Traore.

French forces were lending their support to Mali to "fight against terrorist elements", said the letter, from France's ambassador to the United Nations, Gerard Araud.

The operation would last as long necessary, it added.

Backed by French air power, Malian troops on Friday unleashed an offensive against Islamist rebels who, having seized control of the north of the country in March last year, were threatening to push south.

"Mali faces terrorist elements coming from the north who today threaten the territorial integrity of this state, its very existence and the safety of its population," said the letter.

Addressed to Pakistani ambassador Masood Khan, whose country currently holds the 15-nation panel's presidency, it also called for swift implementation of a Security Council resolution approving an African-led intervention force in Mali.

While the UN Security Council has already approved the 3,000-strong force, it is not expected to be ready to deploy before September.

The letter said that the French operation, "taking place under the framework of international law, will last as long as necessary."