WASHINGTON — The State Department warned Wednesday that a new terrorist group linked to an Algerian militant has emerged as “the greatest near-term threat to U.S. and Western interests” in the Sahel region of Africa. The State Department’s move underscored the resilience of the militant factions and their ability to forge new terrorist alliances, even in the face of Western pressure.

“We are seeing a dangerous mutation of the threat,” said Bruce Hoffman, an expert on terrorism at Georgetown University. “Splinters can become even more consequential than their parent organization.”

The source of much of the concern is Mokhtar Belmokhtar, an Algerian militant who has long been a notorious figure in the Sahel region — a vast area on the southern flank of the Sahara that stretches from Senegal to Chad — and who appears to have become more dangerous even as his ties to Al Qaeda seem to have become more tenuous. Known as Laaouar, or the one-eyed, after losing an eye to shrapnel, Mr. Belmokhtar fought against a Soviet-installed government in Afghanistan.

After returning to Algeria in the 1990s, he joined a militant Algerian group and took refuge in Mali, where he was involved in smuggling and kidnapping for ransom, including the abduction of a Canadian diplomat in 2008.