On Thursday, French military spokesman Col Thierry Burkhard said a column of 1, 400 troops from Chad was heading towards Kidal from the Niger border.

BBC: France's Francois Hollande to visit Mali

Eyewitnesses said French and African troops had left their military base in Gao on Wednesday morning and were heading towards the town of Ansongo, towards the border with Niger, our correspondent adds.

BBC: France action in Mali is real war, says Le Drian

The bodies of the two young Frenchmen, Antoine de Leocour and Vincent Delory, were found on Saturday - a day after the operation on the Niger-Mali border.

BBC: Niger seizes suspects in French hostage 'killings'

The three semi-desert states, which border Niger, Chad and Cameroon, are roughly the size of England or the US state of Illinois but have a population of just 10 million.

BBC: Boko Haram crisis: Nigeria air raids target militants

Last week, southern neighbor Nigeria said it was shutting much of its border with Niger and sending thousands of troops into the border areas in pursuit of a an Islamic insurgency called Boko Haram.

WSJ: Twin Bomb Attacks Kill 20 in Niger

Baga is close to the border with both Chad and Niger and a multinational force from the three countries was attacked, its commander Brig Gen Austin Edokpaye said.

BBC: Baga clashes: Nigeria army seizes heavy weapons

Mali's army chief General Ibrahima Dahirou Dembele told AFP that Chadian troops would join some 500 Nigeri troops in western Niger with the aim of crossing the border and heading towards the town of Gao, in north-eastern Mali.

BBC: Mali conflict: US begins French troop airlifts

He reported on wars in Sudan, Angola and on the Eritrea-Ethiopia border, elections in Algeria and Kenya, Aids in Mali, unrest in the Niger Delta, and conducted one of the BBC's last interviews with Zimbabwean President Robert Mugabe.

BBC: Jeremy Vine