Rival militias clashed in the Libyan capital Tripoli with at least five people killed and 20 wounded, the health ministry said, splintering a four-month-old, UN-brokered ceasefire.
Armoured vehicles were seen in southern Tripoli where militia groups fought for a month until September, part of widespread factional conflict plaguing Libya since the 2011 uprising.
A United Nations-engineered ceasefire agreed then held for the most part until now.
On Wednesday a militia known as the 7th Brigade, or Kaniyat, and a collection of factions called Tripoli Protection Force started fighting in the same southern district.
The 7th Brigade and allied groups triggered the August fighting in a bid to challenge dominance of four “super militias”, including the Tripoli Protection Force, in the streets of the capital.
The UN mission in Libya warned against any breach of the ceasefire.
“Any party initiating a confrontation will be held fully responsible,” it said.
The internationally-recognised government in Tripoli, backed by the United Nations, has been working on a new security plan since the ceasefire but achieved little as Libya lacks a national police force or army.