NATO intervened last year after fighters loyal to Libyan leader Moammar Gadhafi launched attacks on anti-government demonstrators, sparking a civil war that ended in Gadhafi's death in October.
Elections were conducted in the former rebel capital Misurata in February but political momentum was undermined by various restive bids for autonomy in parts of Libya.
The European Union last week called on Tripoli to ensure parties to the conflict in the Sebha region of southern Libya lay down their weapons after at least 70 people were killed in tribal clashes. At least 25 were killed in similar clashes in the west of the country Wednesday.
Ian Martin, U.N. special representative to Libya, said the fighting underscored the need for Libyan authorities to strengthen security institutions and rein in revolutionary fighters.
"The Libyan people sacrificed their lives to secure their freedom for a new, democratic Libya for all its citizens," he said in a statement. "I call on all parties to recall this sacrifice and resolve their differences without resort to the use of force."
Russian Foreign Minister Sergei Lavrov said on the sidelines of talks in Azerbaijan that dealing with the interim government in Libya was growing more difficult.
"The opposition had from the start been under the roof of the transition council, but now that Gadhafi is toppled, they are not particularly easy to talk with," he was quoted by Russia's state-run news agency RIA Novosti as saying.