Eastern Libyan commander Khalifa Haftar has ordered his troops to march on the capital Tripoli, taking his conflict with the internationally recognised government to a dangerous new level.
UN Secretary-General Antonio Guterres, who is in Tripoli trying to push an international peace deal, called for restraint.
When asked about Mr Haftar's statement, he said Libya needed a political not a military solution.
Mr Haftar made his order in a video posted online hours after his forces took full control of Gharyan, a town about 100km south of the capital.
"To our army which is stationed at the outskirts of Tripoli, today we complete our march ... We are going to start shortly," he said in the video titled 'Operation to liberate Tripoli'.
There was no independent confirmation that any advance had started.
The capture of Gharyan after skirmishes on Wednesday with forces allied to Tripoli-based Prime Minister Fayez al-Serraj followed a rapid thrust westwards by Mr Haftar's Libyan National Army (LNA) from his eastern stronghold of Benghazi.
The developments mark a dramatic escalation of a power tussle that has dragged on in Libya since the overthrow of Muammar Gaddafi in 2011.
The country has been divided between the Western-backed government in Tripoli and a parallel administration allied to Haftar since Gaddafi's downfall.
Armed groups from the coastal city of Misrata, who oppose Mr Haftar, started moving towards Tripoli to defend it against the eastern advance, residents said.
There were no immediate reports of clashes - or any statement from the government in Tripoli itself.
The developments are a setback for the United Nations and Western countries which have been trying to mediate between Mr Serraj and Mr Haftar, who met in Abu Dhabi last month to discuss a power-sharing deal.
A national conference is set to follow this month to agree on a road map for elections to resolve the prolonged instability in Libya, an oil producer and a hub for refugees and migrants trekking across the Sahara in the hope of reaching Europe.
Mr Haftar enjoys the backing of Egypt and the United Arab Emirates, which see him as bulwark against Islamists. His opponents see in him as a new Gaddafi.
His forces control the east and recently expanded to southern Libya.
Their advance took diplomats and analysts by surprise while they were focused on neighbouring Algeria, where President Abdelaziz Bouteflika resigned on Tuesday after weeks of protests.
Earlier, one of Mr Haftar's commanders said his forces had taken full control of Gharyan.
"Right now as we speak I'm driving through the town," Abdelsalam al-Hassi told Reuters by telephone.
Gharyan had been allied to the Tripoli government although some Haftar supporters were also based there.
The two sides fought skirmishes on Wednesday but these had ceased, town mayor Yousef al-Bdairi said.
A town resident told Reuters: "The town now is under control of the army who came from the east and I can see their vehicles with their Libyan National Army logo."