“We fear that prolonged conflict will lead to more casualties,” said Syed Jaffar Hussain, WHO’s representative in Libya.
The WHO said it had sent emergency teams to assist frontline hospitals, warning that thousands of people had fled their homes.
UN Secretary General, Antonio Guterres, said: "There is still time to stop violence in Libya and to avoid the worst.” He called for a ceasefire to avoid what he described as a dramatic, bloody battle.
Haftar launched his offensive on Tripoli almost a week ago and his LNA forces appeared to be advancing from the south and south-east of the capital.
“Over the first few days there seemed to be some momentum behind the Libyan army [LNA], although some of it has appeared to have ebbed away more recently,” said Luke Wells, a Libya expert with risk consultancy firm Inkerman Insights.
Q&A: Luke Wells, Inkerman Insights
Heavy clashes took place in the south-eastern town of Ain Zara where fighters loyal to the Government of National Accord pushed back an advance by LNA forces, according to reports from the AFP news agency.
“It was not until a few days after the Libyan army had begun pushing northwards that the Government of National Accord, the internationally recognised administration in Tripoli, announced its counteroffensive and it seems to have been able to mobilise more opposition since then,” Wells told RFI.
The LNA said on Wednesday that it seized a barracks in the Aziziya area, some 50 kilometres south of Tripoli, following a series of ferocious clashes, AFP reported.
Haftar’s forces also held positions in suburbs about 11 kilometres south of the city centre as residents reported LNA aircraft flying over Tripoli and anti-aircraft guns firing at them, according to the Reuters news agency.
“We’re also seeing evidence of Misratan militias, some forces from another powerful western city, moving towards the Libyan army [LNA] to help push them back too,” said Wells.
Haftar’s forces extended their control beyond their eastern stronghold earlier this year, taking the country’s south-western Fezzan region, before launching a long-anticipated push for Tripoli.
Libya has been divided since the 2011 toppling of Moamer Kadhafi with political and armed factions vying for control.