Fresh gunbattles rock Libya capital after brief lull
Clashes flared between the Radaa force and the 444 Brigade in key areas of the city, including the port, the source said.
The fighting eased towards the end of the day, according to television reports and residents who spoke to AFP.
No official casualty figures have yet been released for the latest fighting, but the Libyan Red Crescent said it recovered a dead body from a major street in Tripoli.
The official described the fighting as "urban warfare", with intermittent clashes in residential areas involving light and medium weapons. In other areas, heavy weapons were being used.
Libya has struggled to recover from years of unrest since the NATO-backed 2011 uprising that toppled and killed longtime leader Moamer Kadhafi.
The country remains split between a UN-recognised government in Tripoli, led by Prime Minister Abdulhamid Dbeibah, and a rival administration in the east, controlled by the Haftar family.
The 444 Brigade controls parts of southern Tripoli and is aligned with Dbeibah, whereas Radaa controls parts in the capital's east and holds several key state facilities.
Fighting extended in southern and western Tripoli as Radaa and "groups supporting it came as reinforcements against the 444 Brigade", the interior ministry source said.
On Monday night, heavy arms fire and explosions rocked several Tripoli districts, killing at least six people, according to authorities.
Reports said Abdelghani al-Kikli, leader of the Support and Stability Apparatus which controls the southern district of Abu Salim, had also been killed at a facility controlled by the 444 Brigade.
'Territorial reshuffle'
A source told AFP that groups were moving into the capital from neighbouring Zawiya in support of Radaa.
Meanwhile, "more Misrata brigades may continue to join Dbeibah's side", said Libya expert Jalel Harchaoui.
He described the latest conflict as "more dangerous" for the capital in recent years, saying it meant a "territorial reshuffle" with more factions "seeking to insinuate themselves into downtown Tripoli".
Turkey, a supporter of the Tripoli-based government, on Wednesday called on "all parties to implement a full and lasting ceasefire without delay and to engage in dialogue to settle disputes," its foreign ministry said.
On Tuesday, the Tripoli-based government said the fighting had been brought under control as Dbeibah thanked government forces "for restoring security and asserting the state's authority in the capital".
Dbeibah also announced a string of executive orders including dissolving some bodies previously run by Tripoli armed groups other than the 444 Brigade.
But a second night of fighting could mean "a more prolonged, destructive, and existential battle with a nationwide dimension" after what he said was Dbeibah's "failure to secure a quick victory".
Authorities also announced a ceasefire, but gunshots were still heard in western parts of Tripoli.
The United Nations mission in Libya said it was "deeply alarmed by escalating violence in densely populated neighbourhoods of Tripoli for the second night in a row".
In a statement, it called for "an immediate, unconditional ceasefire in all areas, allowing safe corridors for the evacuation of civilians trapped in intense conflict zones".