LAGOS: Nigeria has not sent troops to Mali despite claims that it was backing the country's army in an offensive against Islamists who control the country's north, a defence spokesman said Saturday.

"We have not sent anything," Colonel Mohammed Yerima told AFP. "Nothing. We have responded that we are going to send troops."

Yerima could not provide further details on when troops might be deployed or how many. Nigeria has previously spoken of sending 600 troops to take part in an African force to help Mali's army win back the country's north.

A spokesman for Nigerian President Goodluck Jonathan did not immediately respond to requests for comment.

Mali's army said Nigerian and Senegalese forces were involved in Friday's offensive which was backed by French air power. Dakar has however also denied having any troops there.

The UN Security Council has approved a 3,000-strong force, but it is not expected to be ready to deploy before September.

The chairman of the west African bloc ECOWAS on Friday authorised the immediate deployment of troops, but gave no further details on how or when it would occur.

In a statement on Saturday, the head of the ECOWAS commission said "measures will be taken to implement this decision."

Nigeria has the largest military in the 15-nation Economic Community of West African States, but it is also battling an insurgency by Islamist extremist group Boko Haram in its northern and central regions.

The Islamists' advance in Mali has raised fears that the country could provide a safe haven for Al Qaeda-linked extremists, posing a threat to the region as well as Europe and beyond.