Updated

The closer the U.S. gets to its original goal in Syria of defeating the Islamic State group, the murkier its end game becomes.

New layers of complexity are descending on a shifting battlefield, as demonstrated by a deadly barrage of American air and artillery strikes on a shadowy attacker.

The Pentagon insists it is keeping its focus on defeating IS, but Defense Secretary Jim Mattis says U.S.-backed fighters in eastern Syria faced a "perplexing" overnight assault by about 300 pro-Syrian government fighters whose nationalities, motives and makeup he could not identify.

A number of U.S. military advisers were present alongside local allied forces, and the Americans led a punishing response that killed about 100 of the assailants.

Mattis says the incident doesn't portend a deeper involvement in Syria's civil war.