UN warns against new clashes in Libya capital
"The United Nations Support Mission in Libya (UNSMIL) strongly condemns the recent military mobilisation of forces in southern Tripoli and is monitoring the situation closely," it said in a statement.
"UNSMIL warns parties against any breach of the ceasefire agreement concluded on 4 and 9 September 2018."
Fierce battles between militias from the capital and from other areas in the west of the country rocked Tripoli between the end of August and early September, leaving at least 117 dead and hundreds wounded.
The UN brokered the September truce deal that helped end the violence.
But witnesses reported sporadic shooting and road closures on Wednesday after the Tripoli Protection Force alliance warned on Facebook it would "repel an attack" by a rival group that had failed to withdraw from the capital.
The Seventh Brigade militia, from the town of Tarhuna, has maintained its positions in Tripoli since the ceasefire agreement.
The UN said that "any party initiating a confrontation will be held fully responsible" and insisted it "will take action as necessary" to halt any violence.
Libya has been torn between rival administrations and a myriad of militias since the overthrow and killing of dictator Muammar Gaddafi in 2011.
The main armed groups in Tripoli say they are loyal to the internationally-backed Government of National Accord (GNA), but officials have struggled to exert real control over the fighters.
The GNA announced security reforms in the wake of the bloodshed last year, aimed at curbing the power of militias in the capital.
A second administration in the east of Libya is backed by militia leader Khalifa Haftar and his self-proclaimed Libyan National Army.
The UN is pushing for elections to help stabilise Libya, but chronic violence has delayed plans for a vote.