Libya has been plunged into chaos again, with forces loyal to a polarizing military commander marching on the capital and trading fire with militias aligned with a weak U.N.-backed government.

Casualties were mounting in fighting that threatens to disrupt U.N. plans for an election to end rivalries between parallel administrations in Libya.

The military thrust has marked a dangerous escalation of a power struggle that has dragged on since the overthrow of Muammar Gaddafi in 2011.