An air strike late on Tuesday hit a detention centre for mainly African migrants in a suburb of the Libyan capital of Tripoli, killing at least 40 people and wounding 80, a health official said.

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.