The UN Support Mission in Libya said Tuesday that it was "gravely alarmed" over new military mobilizations by rival groups in the Libyan capital.
In recent days, armed vehicles have reportedly moved into Tripoli from Misrata, around 200 kilometers (125 miles) away, and on Monday night gunfire was heard in the capital, according to witnesses and videos circulated online.
UNSMIL said the recent "rapidly escalating tensions and military mobilization" in Tripoli "could lead to armed confrontation".
Sabiha Mohamad, a resident in western Tripoli, told AFP she had heard the gunshots but said "there was more fear than harm".
There were no official reports of casualties, and it remained unclear who fired the shots.
"We stay cautious when going out, because anything could break out at any moment," Mohamad said.
Libya is split between the Tripoli-based Government of National Unity (GNU), led by Prime Minister Abdulhamid Dbeibah, and a rival administration in the east led by Osama Hammad.
The North African country has remained divided since a NATO-backed revolt toppled and killed longtime leader Moammar Gaddafi in 2011.
UNSMIL urged "all actors to cease any preparations for violence", warning they could spiral into a "'lose-lose' war".
In May, Tripoli was rocked by days of deadly fighting between rival armed groups that left at least eight people dead, according to the UN.
The fighting came after Dbeibah announced a string of executive orders seeking to dismantle armed groups that he said had "become stronger than the state".
Tensions have been flaring between government-aligned groups and Radaa, a powerful rival faction that controls Mitiga airport and other parts of eastern Tripoli.
Last month, UNSMIL chief Hanna Tetteh proposed a roadmap for "general elections and the unification of institutions" in the divided country.
The plan looks to implement a "politically viable electoral framework" and "a new unified government", the UN said.
