Libyan strongman Khalifa Haftar ordered his troops Thursday to "advance" on Tripoli, seat of the internationally-recognised unity government, after UN chief Antonio Guterres warned against a major flare-up.
"The time has come," Haftar said in an audio message released online by his self-proclaimed Libyan National Army.
Haftar's forces announced Wednesday they were gearing up for an offensive in the west of the country to purge it of "terrorists and mercenaries".
Following that statement, a convoy of LNA vehicles pushed towards the city of Gharyan, some 100 kilometres (60 miles) from Tripoli, witnesses and military sources said.
Commander Abdessalem Al-Hassi told AFP Thursday that his forces had entered into the city without a fight.
But at least four sources in the city denied this, and a Gharyan official said there were "ongoing efforts to avoid a confrontation" between rival fighters who divide the city.
Dozens of militias have fought for control of the North African country since a NATO-backed uprising toppled and killed longtime dictator Moamer Kadhafi in 2011.
Haftar's forces have emerged as a key player, opposing the government in Tripoli and backing a parallel administration in the east.
Unity government chief Fayez al-Sarraj condemned Haftar's "escalation" and said he had ordered loyalist forces to prepare to "face all threats".
His interior ministry announced a "state of maximum alert" and powerful armed groups from Libya's western city of Misrata said they were ready to halt Haftar's advance.
The rise in tensions came as United Nations chief Guterres visited Tripoli ahead of a planned conference later this month to hammer out a roadmap for delayed parliamentary and presidential elections.
"I am deeply concerned by the military movement taking place in Libya and the risk of confrontation," the visiting Guterres tweeted from the capital.
"There is no military solution" to Libya's woes, he added.
Haftar's forces in January launched another offensive into oil-rich southern Libya, ostensibly aimed at wiping out "terrorists" and criminal groups.
He has repeatedly expressed his intention to march on Tripoli.
The European Union warned that the "military buildup underway in Libya and the escalatory rhetoric... seriously risks leading to an uncontrollable confrontation".
The United States' embassy said it "strongly condemns the increase in violence in Libya and reiterates the UN's call for restraint".
Rival leaders had agreed in Paris last year to hold elections before the end of the year, but that vote never materialised as Haftar's forces and the Tripoli government grappled for power.
The UN said Haftar and Sarraj agreed at a meeting in Abu Dhabi last month to make a new attempt to organise the elections.
Libyan analyst Emad Badi said factions in western Libya were angling for a joint front against the strongman, but "whatever the result ... Haftar will benefit from it militarily or politically".
"His progress can enhance his position in the negotiations" if the UN-backed conference set for this month goes ahead, Badi added.
Jalel Harchaoui, a researcher at the Clingendael Institute in The Hague, said that "taking Tripoli is certainly not an easy or risk-free task, but it's still a possibility."
"The risk of conflagration has increased," he said.