The rebels are on the outskirts of the capital and say they have seized Tripoli's global airport.
Libya's unity government was created at UN-backed talks in 2015 but it has struggled to assert control, while a number of global initiatives since have failed to unite the country.
Rebel forces are advancing on Tripoli in a multi-pronged attack from the south and west of the city, although they have reportedly been slowed by pro-government fighters.
The escalation comes after forces commanded by Haftar, who runs the self-styled Libya National Army based in the country's east, pushed westward.
Libya has been riven by violence and division since long-time ruler Muammar Gaddafi was deposed and killed in 2011.
Haftar has ordered his troops to continue their march on Tripoli.
"We have extended our hands towards peace but after the aggression that has taken place on the part of forces belonging to Haftar. he will find nothing but strength and firmness", he said. Haftar, as the military leader in the east, may not have yet secured the level of militia support he expected inside Tripoli, but one source said alliances are shifting by the hour, with reports of violent incidents often exaggerated on social media.
"We were stabbed in the back", he said Saturday in televised comments, adding that his forces would confront Hifter's troops with "force and determination". The LNA vowed to retaliate.
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Tripoli residents have expressed concern that large-scale fighting could break out and began stocking up on food and petrol. It also urged all Libya forces "to de-escalate and halt military activity". "The situation is very worrying and we can not accept a further military escalation", Maas said after a ministerial meeting.
Russia's Foreign Minister Sergey Lavrov says his country is opposing unilateral attempts to determine who is to blame in Libya.
The United Nations aims to stage a conference in the southwestern town of Ghadames on April 14-16 to weigh elections as a way out of the country's factional anarchy, which has seen Islamist militants establish a toehold in some areas. But they failed to detail enforcement mechanisms if the Libyan strongman refused to commit to a political solution.
United States special forces were sent to Libya several times in recent years, with a series of raids to capture Libyans accused of terrorism. Each is backed by an array of militias.
Despite the chorus of global concern over his actions, he has had support from powerful outside players, including the UAE and Egypt.
Few thought he would go ahead and launch this operation - which he has long threatened to do - because they believed ongoing talks that saw him go from Paris to Palermo and the UAE for more than a year would buy time until a new political settlement was reached through negotiations and an eventual electoral process. Haftar has recently seized much of southern Libya without fighting.
The Prime Minister of Libya Fayez al-Serra was filmed preparing the defence of the city.