Deadly clashes broke out Saturday in Libya's capital Tripoli between militias backed by its two rival administrations.
At least 23 people were killed, including 17 civilian, and 140 more wounded, the Health Ministry said.
Six hospitals were hit and ambulances were unable to reach areas affected by the fighting.
The escalation threatens to shatter the relative calm Libya has enjoyed for most of the past two years
The clashes involved groups that back the Tripoli-based Government of National Unity (GNU), led by Abdulhamid Dbeibah, against supporters of former Interior Minister Fathi Bashagha, backed by the House of Representatives in the eastern city of Tobruk.
The UN mission in Libya said the fighting involved "indiscriminate medium and heavy shelling in civilian-populated neighborhoods” of Tripoli.
It came a month after a bout of fighting shook the relative peace in the Libyan capital, killing several civilians.
The two factions have repeatedly mobilized in Tripoli in recent weeks. This week, Dbeibah-linked factions paraded around Tripoli.
A GNU statement blamed Saturday's clashes on pro-Bashagha forces, saying they had fired on a convoy in Tripoli. Other pro-Bashagha units had amassed outside Libya's capital, according to the statement.
Bashagha's administration did not directly respond to the accusation.
Bashagha tried to enter Tripoli in May, and was then driven out following hours of fighting.
The fighting follows rising tensions between the rival administrations.
Bashagha, who is backed by the Tobruk-based parliament, says the Dbeibah-run GNU's mandate has expired.
Bashagha was appointed in February by the parliament, which was elected in 2014. He has been unable to impose authority in Tripoli.
Dbeibah was appointed by a UN-backed commission last year on an interim basis. He had vowed to cede his position to a democratically elected leader, but the elections he had pledged to hold last December failed to materialize.
The GNU said negotiations for the calling of new elections had been underway, but Bashagha "walked out at the last moment."
Meanwhile, Bashagha's administration says it has never rejected talks with Dbeibah.
Libya has been marked by sometimes violent power struggles between different factions since the 2011 overthrow of strongman Moammar Gadhafi, who ruled the North African country for 42 years.