"The United Nations continues to call for a temporary humanitarian truce to allow for the provision of emergency services and the voluntary passage of civilians, including those wounded, from areas of conflict," Maria do Valle Ribeiro, the UN humanitarian coordinator for Libya, said in a statement.
The clashes south of the capital have killed at least 25 people, including fighters and civilians, and wounded 80, a spokesman for the Tripoli-based Health Ministry told Reuters.
An airstrike on Tripoli's main Mitiga airport forced its shutdown and stranded passengers. It was the capital's only functioning airport.
Hours later, the UN special envoy in Libya, Ghassan Salamé, condemned Hifter's forces for the airstrike, declaring that "this attack constitutes a serious violation of international humanitarian law, which prohibits attacks against civilian infrastructure."
The 75-year-old Hifter, a US citizen who lived in exile in Northern Virginia for years, is aligned with a rival government based in the eastern city of Tobruk. He returned to Libya during the 2011 revolution and subsequent Nato intervention that toppled dictator Muammar Gaddafi and led to his killing.
Hifter has long coveted Tripoli, where Libya's oil revenue flow to and where the central bank - which pays the salaries of soldiers and civil servants - is based. In the past few months, Hifter's forces - a constellation of militias calling themselves the Libyan National Army - have swept through the south and quickly pushed north towards Tripoli.
The tensions in Tripoli are the worst since conflict erupted between Hifter and rival militias in 2014 and threaten to plunge the country into a full-blown civil war.
A counteroffensive by the UN-installed and Western-backed Tripoli government has slowed Hifter's advance. The Tripoli government, with no army of its own, has relied on dozens of militias to remain in control, including many from other cities, particularly Misurata and Zintan.