The UN envoy to Libya Ghassan Salamé has expressed his growing concern that the battle for the capital Tripoli is escalating.
Mr Salamé described the current situation as a military stalemate.
But he said both sides in this battle for Tripoli were now mobilising resources and both still believed they could win militarily.
In the past few days, the Libyan government and Gen Khalifa Haftar’s forces have launched air strikes against each other.
Mr Salamé told the BBC his worst fear now was that foreign powers may become directly involved with more advanced weaponry.
Gen Haftar has long received military backing from a number of Arab states as well as France while the government is said to be receiving military support from countries including Turkey and Qatar.
This upsurge in fighting has shattered UN plans for a political solution to this turmoil which has fractured Libya since Muammar Gaddafi was ousted in 2011.
The human cost of this conflict is also mounting.
Mr Salamé said that, so far, fighting was not focused on residential areas of the capital, but if this escalation continues, casualties would be, in his words, of “a completely different magnitude” in the city where nearly a third of Libya’s population lives.
The UN says that In the last 10 days of fighting, more than 130 people were killed, more than 600 were injured, and thousands have been displaced.
The latest upsurge started when on 4 April the forces of Gen Haftar, which dominate large swatches of Libyan territory, launched an offensive to take the capital Tripoli which is controlled by the UN-backed government.