A resumption in fighting in Libya is threatening to unravel efforts to end a nearly decadelong crisis in the North African country, less than two weeks after foreign backers of the two main warring factions agreed to support a cease-fire there.

Forces loyal to Khalifa Haftar, a former military commander who leads a faction based in the country’s east, shelled the Libyan capital’s only functioning airport this week as they attempted to take control of Tripoli from the United Nations-backed government.

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