Libyan army commander Khalifa Haftar in an audio recording posted online has ordered his forces to march to Tripoli, the capital of the UN-backed government, provoking criticism from Turkey, US and allies who warned of consequences.
Haftar, who commands the so-called Libya National Army [or LNA] based in the east, described his forces' move as a "victorious march" to "shake the lands under the feet of the unjust bunch."
He ordered forces not to open fire on any civilians saying, "whoever raises the white banner is safe."
His forces have taken over the town of Gharyan, 50 km from Tripoli.
"The clock had struck and it's high time. It's our date with the great conquest, so advance with trusted steps, and enter Tripoli in peace with those who want peace, as supporters of truth, not invaders," he said.
"Use your weapons only against those who prefer to confront and fight you, and only open fire when you are shot at."
His forces took full control of Gharyan on Thursday.
The taking of Gharyan after skirmishes on Wednesday with forces allied to Tripoli Prime Minister Fayez al Serraj culminated a rapid thrust westwards by Haftar's forces from his eastern stronghold of Benghazi.
The developments signified a serious escalation of the conflict in Libya that has dragged on since the overthrow of Muammar Gaddafi in 2011.
"We fully control Gharyan and right now as we speak I'm driving through the town," Abdelsalam al Hassi, commander of the operation to secure western Libya, told Reuters news agency by telephone.
Sarraj earlier 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.
Britain requests urgent UNSC meeting
Britain on Thursday requested an urgent UN Security Council meeting on Libya, diplomats said.
The council is expected to hold a closed-door meeting on Friday following the order from Haftar.
Turkey warns against harming national accord
Turkey is closely following the recent military escalation in the west of Libya, and it's "important to avoid attempts that would harm the spirit of national accord," Foreign Ministry spokesman, Hami Aksoy, said.
"We believe that the present division in Libya can only be overcome through intra-Libyan dialogue," it said.
Turkey said it supports the UN-facilitated National Conference which is planned to take place in Libya's Ghademes on April 14-16.
US and allies seek halt in Tripoli march
The governments of France, Italy, the United Arab Emirates, Britain and the United States said on Thursday they were deeply concerned about fighting around the Libyan town of Gharyan and urged all sides to immediately de-escalate tensions.
"At this sensitive moment in Libya's transition, military posturing and threats of unilateral action only risk propelling Libya back toward chaos," they said in a joint statement released in Washington by the State Department.
"We strongly believe that there is no military solution to the Libya conflict.
UN urges restraint
UN Secretary-General Antonio Guterres meanwhile appealed for restraint by the rival factions.
Guterres, who arrived in Tripoli on Wednesday to promote peace talks, said in a tweet before Gharyan's capture that he was deeply concerned by the military movements and the risk of confrontation.
"There is no military solution.
Only intra-Libyan dialogue can solve Libyan problems. I call for calm and restraint as I prepare to meet the Libyan leaders in the country," he said.
Libya has been divided between the internationally recognised government in Tripoli and a parallel administration allied to Haftar since Gaddafi's ouster.
Gharyan, lying in the western mountains about 100 km due south of the capital, had been allied to the Tripoli government.
The two sides fought skirmishes on Wednesday but these had ceased, town mayor Yousef al Bdairi said earlier on Thursday.
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."
The developments are a setback for the United Nations and Western countries which have been trying to mediate between Serraj and 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.
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.
There was no immediate comment from the Tripoli government, which has issued a general alert for its forces in response to the eastern advance.
Analysts doubt the LNA is capable of launching a full-scale attack as it has stretched itself with the southern advance and it also relies on tribesmen and other auxiliary forces.