Mrs. Clinton made her remarks at a special United Nations meeting on the political and security crisis in the parts of North Africa known as the Maghreb and the Sahel, particularly in northern Mali, which has been overrun by Islamic extremists since a military coup helped lead to the division of that country this year.

Al Qaeda in the Islamic Maghreb has long operated in the region, she said, and was now exploiting a haven in Mali to export extremism and terrorist violence to neighbors like Libya.

“Now with a larger safe haven and increased freedom to maneuver, terrorists are seeking to extend their reach and their networks in multiple directions,” Mrs. Clinton told leaders assembled at the meeting, including President François Hollande of France and the United Nations secretary general, Ban Ki-moon. “And they are working with other violent extremists to undermine the democratic transitions under way in North Africa, as we tragically saw in Benghazi.”