Mali's president has asked France for help countering an offensive rebel groups who control the northern half of the country and are heading south.

France's UN Ambassador Gerard Araud told reporters after an emergency meeting of the UN Security Council late on Thursday that urgent action was needed against the groups who captured the city of Konna on Thursday and are currently threatening the city of Mopti, which has 100,000 inhabitants.

"This terrorist attack weakens even more the stability of Mali and thereby that of its neighbours," Araud said. "Sustainability of the Malian government and the protection of civilian populations are now at stake."

Araud said Mali's President Dioncounda Traore sent a letter to Secretary-General Ban Ki-moon, which he transmitted to the Security Council, and a similar letter to French President Francois Hollande seeking assistance from France, the country's former colonial power, against the offensive.

"It's up to the authorities of my country to decide and announce the nature of this assistance," Araud said. "It will be announced in Paris tomorrow."

The Security Council expressed "grave concern" at the military action by the terrorist and extremist groups and called on UN member states "to provide assistance to the Malian Defence and Security Forces in order to reduce the threat posed by terrorist organisations and associated groups".

Security vacuum

Araud said council resolutions "call on all member states to provide assistance in resolving the Malian crisis in all its aspects, including military and political - and I emphasise - to provide support to the authorities of this country to put an end to the terrorist threat.''

Mali was plunged into turmoil after a coup in March 2012 created a security vacuum. That allowed the Tuaregs, who have long felt marginalised by Mali's government, to take half the north as a new homeland.

But months later, the rebels were kicked out by groups allied with al-Qaeda, which have imposed strict Shariah law throughout the north.

Late last year, the 15 nations in West Africa, including Mali, agreed on a proposal for the military to take back the north, and sought backing from the United Nations.

The Security Council in December authorised an African-led force to support Malian forces in recovering the north - an area the size of Texas - but set no timeline for military action.

Instead, it set out benchmarks to be met before the start of offensive operations, beginning with progress on a political roadmap to restore constitutional order, political reconciliation, elections and training of the Malian and African troops and police.

The Security Council called for "the immediate issuance of an agreed political roadmap, which includes serious negotiations with non-extremist Malians in the north and presses for the full restoration of democratic governance".