TRIPOLI, Sept 3 (Reuters) - Fighting broke out in Tripoli early on Friday between rival armed forces, the heaviest clashes in the Libyan capital since the conflict between eastern and western factions paused a year ago. A resident of the Salah al-Din district in southern Tripoli said shooting began at about 2.30 a.m. and continued through the morning with medium and light weapons. Conflict in Tripoli between the armed groups who vie to control both territory and state institutions would further undermine the prospect of December elections as part of a plan to end a decade of chaos, violence and division. Despite a ceasefire and progress earlier this year towards a political solution to Libya"s crisis, there has been no movement towards integrating its myriad armed groups into a unified national military. The new fighting pitted the 444 Brigade against the Stabilisation Support Force, two of the main forces in Tripoli, a witness said. The head of the Tripoli Military Zone, a structure set up to organise the various armed forces in the city during the civil war, indicated that the fighting was aimed at curbing the activities of 444 Brigade. "What happened is to correct the brigade"s deviation from its course and non-compliance with military orders," Abdulbaset Marwan said in a video statement. The 444 Brigade told Reuters it had been "surprised by an assault by armed men" and said it was surprised at Marwan"s statement. The United Nations Libya mission called for an immediate halt in the fighting, saying it had "grave concern". VIOLENCE Libya is a major oil producer and though it has been able to maintain output over the past decade, disputes have sometimes shut down exports, including for months last year. The fighting follows major clashes last month in the city of Zawiya, west of Tripoli, and smaller incidents of friction or clashes inside the capital including a gunfight this week at a state institution. In eastern Libya, controlled by Khalifa Haftar"s Libyan National Army (LNA), there have also been shootings and other incidents of violence in recent months. Libya has had little peace since the 2011 NATO-backed uprising that overthrew longtime dictator Muammar Gaddafi, and it divided in 2014 between warring eastern and western factions. However, they agreed a ceasefire last year and a new unity government that both sides backed was installed in March to prepare for national elections in December, moves seen as the best chance for peace in years. The Tripoli-based unity government has however struggled to unify state institutions or prepare for elections, with the eastern-based parliament rejecting its budget and failing to agree a constitutional basis for a vote. Political factions have squabbled repeatedly over the role and powers of the interim government as well as over the control of state institutions and the public purse. Wolfram Lacher, of the German thinktank SWP, said that although there was the possibility of further escalation, a mediated solution was likely to resolve the fighting in the short term. However, "similar clashes are bound to recur in Tripoli and elsewhere", he added.