Libyan military commander Khalifa Haftar ordered his force to advance on the capital on Thursday to clear Tripoli of militias, according to a recording of a speech to his troops.
“We hear your call, Tripoli,” Field Marshal Haftar told his Libyan National Army. “It is now the time for the great victory. March forward.”
Meanwhile, UN Secretary General Antonio Guterres on Thursday made an urgent call for peace as the force pushed closer to Tripoli.
Mr Guterres, visiting the capital, has urged all sides to end fighting and concentrate on a peace conference being organised by the UN this month.
“There is no military solution,” he said. “Only intra-Libyan dialogue can solve Libyan problems.”
Armoured columns from the LNA pushed towards the Libyan capital from the east and the south-west.
The troops were gathering “to liberate the homeland from terrorism", Ahmed Al Mesmari, the army's spokesman, told Reuters.
Field Marshal Haftar’s forces captured the two eastern town of Benghazi and Derna from militias in the past two years, both after months of fighting.
The LNA published a video on Wednesday showing a convoy of armoured vehicles on the road.
"In fulfilment of Field Marshal Haftar's orders, several military units moved to the western region to purge the remaining terrorist groups in their last hideouts,” a message on the video said.
Reports from witnesses indicate the LNA is moving towards Tripoli along the coastal highway from eastern Libya, and from south-west Libya.
The southwest force was engaged in clashes on Wednesday at Asabiah, about 110 kilometres south of the capital, with one person killed.
On Thursday, the LNA said its troops had peacefully taken control of Gharyan, a city 90km south of the capital.
The LNA – which has a regular army, navy and air force – has had success under Field Marshal Haftar in recent years.
In September 2016 it captured the key ports of the eastern region, which has two thirds of Libya’s oil production.
In January it launched a successful offensive to capture south-west Libya, with most of the region falling under its control by late March. Some towns welcomed the LNA troops.
In other areas, the army used its air power for strikes and long-range reconnaissance.
In Tripoli, the UN-backed Government of National Accord responded to LNA advances on Wednesday by issuing a “general alert”.
Security in Tripoli depends on rival militias who periodically fight one another, and the government’s general alert is an attempt to unite them against the LNA.
Some in Libya say lack of support for the government is caused by its inability to provide basic services.
"The escalation is a direct result of the failure of the GNA to achieve anything in Libya," said an editorial tweet from Fezzan Libya Org, a south Libyan news website.
“It has had all the power and money to do good but decided to keep the money for itself.”
Supporters of the GNA, which was installed in Tripoli in 2016, point to its successes, including a crackdown on corruption, and say its effectiveness has been hampered by the chaos in western Libya.
There is some uncertainty about whether other Libyan forces will be drawn into the fighting.
The western city of Misrata is home to powerful militias allied to the government, which has yet to decide whether to fight Field Marshal Haftar's army.
In the north-west, strong forces in Zintan are allied to the LNA, but have also yet to announce a stance.
UN officials are concerned that fighting will force cancellation of the national conference, a gathering of Libya’s factions and political leaders, due to be held from April 14-16 in the western city of Ghadames.
The conference is the brainchild of UN special representative Ghassan Salame, who last year tried and failed to persuade Libya to hold elections.
Months of patient diplomacy by Mr Salame has produced a fragile consensus on the need for the conference, which diplomats hope will produce agreement among Libya's leaders on the need for elections this year.
Crucial to the success of the conference will be if its decisions are backed by the House of Representatives parliament in Tobruk, which runs a rival government to the GNA and supports Field Marshal Haftar.
The US and its allies called Thursday for an immediate de-escalation and warned that military action would have consequences.
"Our governments oppose any military action in Libya and will hold accountable any Libyan faction that precipitates further civil conflict," said a joint statement by the UAE, US, France, Britain and Italy.
The governments said they were deeply concerned by fighting near Gharyan, and urged all parties to immediately stand down.
"At this sensitive moment in Libya's transition, military posturing and threats of unilateral action only risk propelling Libya back toward chaos," they said.
"We strongly believe that there is no military solution to the Libya conflict."
Following a request from Britain, the UN Security Council will meet behind closed doors at 3pm on Friday to discuss the situation.