DAKAR, Senegal — Just as Al Qaeda once sought refuge in the mountains of Tora Bora, the Islamist militants now on the run in Mali are hiding out in their own forbidding landscape, a rugged, rocky expanse in northeastern Mali that has become a symbol of the continued challenges facing the international effort to stabilize the Sahara.

Expelling the Islamist militants from Timbuktu and other northern Malian towns, as the French did swiftly last month, may have been the easy part of retaking Mali, say military officials, analysts and local fighters. Attention is now being focused on one of Africa’s harshest and least-known mountain ranges, the Adrar des Ifoghas.

The French military has carried out about 20 airstrikes in recent days in those mountains, including attacks on training camps and arms depots, officials said. On Thursday, a column of soldiers from Chad, versed in desert warfare, left Kidal, a diminutive, sand-blown regional capital, to penetrate deep into the Adrar, said a spokesman for the Tuareg fighters who accompanied them.

“These mountains are extremely difficult for foreign armies,” said the spokesman, Backay Ag Hamed Ahmed, of the National Movement for the Liberation of Azawad, in a telephone interview from Kidal. “The Chadians, they don’t know the routes through them.”