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 (LNA) came as UN Secretary General Antonio Guterres was visiting Tripoli.
Mr Guterres spoke of his concern and urged an end to the escalation.
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 deep divisions since its long-time ruler Muammar Gaddafi was deposed and killed seven years ago.
The UN had been planning to hold a conference in Libya later this month for talks over ending the country’s long-running crisis.
There were conflicting reports that Gen Haftar’s forces had entered the town of Gharyan, 100km (60 miles) south of Tripoli.
The LNA says it has secured Gharyan and moved on.
The internationally recognised and UN-backed government in Tripoli said it had put its forces on high alert.
In December Gen Haftar met Prime Minister Fayez al-Serraj from the UN-backed government at a conference but refused to attend official talks.
Gen Haftar has received backing from Egypt and the UAE.