Militia clashes close to oil ports in the Sirte Basin have brought production in Libya’s oil heartland to a near standstill. The latest escalation pushes the country further towards civil war, and raises the spectre of a prolonged output slump.

Fighting near key export terminals slashed Libyan oil output this week, as a deepening political divide finally caught up with the country’s energy sector.

Islamist militia in control of the capital Tripoli thrust eastwards on 15...