Eastern Libyan forces carried out air strikes on the southern part of Tripoli on Sunday and made progress towards the city centre, residents said, escalating an operation to take the capital as the United Nations failed to achieve a truce.
The Libyan National Army (LNA) force of Khalifa Haftar, which backs a parallel administration in the east, last week launched an advance on Tripoli in the west, home to the internationally recognised government.
"The air force took part for the first time in the military operations," said LNA spokesman Ahmed Mismari. "It conducted a very successful operation to secure the airport road (to city centre)," he added.
The UN mission to Libya (UNSMIL) called on Sunday for a truce for two hours in southern Tripoli to evacuate civilians and wounded, it said in a statement without giving details.
But the true was not observed by evening, one UN official said.
Haftar, 75, who casts himself as a foe of Islamist extremism but is viewed by opponents as a new dictator in the mould of Gaddafi, enjoys the backing of Egypt and the United Arab Emirates, which see him as a bulwark against Islamists and have supported him militarily, according to UN reports.
"No justification for LNA move on Tripoli," UK Foreign Minister Jeremy Hunt tweeted, adding he was watching the situation very closely and would discuss the "next steps" with the European Union on Monday.
In the past, Haftar has struck deals with armed factions outside Tripoli to advance his forces. But gaining control of Tripoli - the ultimate prize for Haftar s eastern parallel government - would be far more complicated.
Armed groups allied to the Tripoli government have moved more machinegun-mounted pickup trucks to defend Tripoli from Misrata down the coast. Misrata is known for a spirit of resisting "old regime" figures, developed during 2011 when pro-Gaddafi forces besieged it for three months.