Gunfire and explosions echoed through Tripoli in the early hours of Friday as eastern Libyan forces pushed against militias backing the internationally recognised government around the disused international airport and the Ain Zara district.
Fighting between Field Marshal Khalifa Haftar's Libyan National Army, which is allied to an administration based in eastern Libya, and troops loyal to the Tripoli government of Prime Minister Fayez Al Sarraj has displaced 9,500 people in the capital, the United Nations said.
The World Health Organisation said it had made contingency plans in case "thousands if not hundreds of thousands" were displaced.
After a week of fighting, 75 people have been killed and 323 wounded, including seven civilians killed and 10 wounded, Dr Syed Jaffar Hussain, WHO representative in Libya, told a Geneva news briefing by telephone from Tripoli.
Dr Hussain said he feared outbreaks of tuberculosis, measles and diarrhoeal diseases because of poor sanitation, especially among the displaced.
He said the WHO had delivered trauma kits and medicines to hospitals in Tripoli. "These supplies will last for two weeks, the acute phase," he said.
Field Marshal Haftar's push on Tripoli in Libya's north-west is the latest turn in a cycle of factional violence and chaos dating back to the 2011 uprising that overthrew dictator Muammar Qaddafi.
After sweeping up from the desert south in early April, the LNA has been held up in the southern suburbs of Tripoli, about 11 kilometres from the centre.
The UN humanitarian agency Ocha said 3,500 people had left their homes in Tripoli in the previous 24 hours, and that 90 per cent of those who had requested evacuation could not be moved to safer areas.
Late on Thursday, the European Union urged the LNA forces to stop their offensive.
As well as the toll on civilians, the renewed conflict threatens to disrupt oil supplies, increase migration across the Mediterranean to Europe, derail a UN peace plan and encourage Islamist militants to exploit the chaos. Libya is a main transit point for migrants who have poured into Europe in recent years, mostly trafficked by smuggling gangs.
The LNA forces swept out of their stronghold in eastern Libya to take the sparsely populated but oil-rich south earlier this year before heading towards Tripoli, where Mr Al Sarraj's UN-backed Government of National Accord sits.
Libya's National Oil Corporation chief Mustafa Sanallah told the government in a meeting on Thursday that the latest fighting poses the greatest threat to the country's vital oil and gas exports since 2011, according to a statement issued by the GNA.
Field Marshal Haftar has so far resisted UN pressure to accept a power-sharing settlement to stabilise Libya, using his leverage as an ally of the West in attempts to stem Islamist militancy in North Africa.