The leader of forces in eastern Libya has ordered them to march on the capital Tripoli, the base of the internationally recognised government.
Khalifa Haftar's order to the self-styled Libyan National Army came as UN chief Antonio Guterres was in Tripoli.
Armed groups from the western city of Misrata, which back the government, have vowed to stop any advance.
Libya has been riven by violence and division since long-time ruler Muammar Gaddafi was deposed and killed in 2011.

What is happening on the ground?
After Gen Haftar's announcement, his forces moved towards the capital from several directions, one of his spokesmen said.
There were conflicting reports that Gen Haftar's forces had entered the town of Gharyan, 100km (60 miles) south of Tripoli.
The Libyan National Army (LNA) says it has secured Gharyan and moved on. However it said two of its soldiers had been wounded in clashes in a nearby area.
A Gharyan official told AFP that there were "ongoing efforts to avoid a confrontation" between rival fighters in the town.
The UN-backed government in Tripoli said it had put its forces on high alert.
Meanwhile residents in Misrata said armed groups from the city had begun moving towards the Libyan capital, Reuters reported.

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