The number of people fleeing violence in West Africa's Sahel region has quadrupled in the past two years, with 2 million now displaced in their own countries, the U.N. refugee agency UNHCR said on Friday.

Militants linked to al Qaeda and Islamic State have expanded their reach in the semi-arid region on the edge of the Sahara, stoking ethnic conflict in Burkina Faso, Mali and Niger and forcing whole communities to flee their homes.

More than half of those displaced within their own country are in Burkina Faso, where many are forced to sleep outside and do not have enough water, UNHCR spokesman Boris Cheshirkov said at a briefing in Geneva.

"The communities hosting the displaced have reached a breaking point," he said. "The humanitarian response is dangerously overstretched, and UNHCR is urging the international community to redouble its support for the region."

The Sahel also has more than 850,000 refugees who have fled across international borders, mainly from Mali.