The death toll - higher than that given by either side yet - appeared to be mainly fighters, although it also included some civilians including two doctors, WHO spokesman Tarik Jasarevic told a Geneva news briefing.
The eastern Libyan National Army (LNA) forces of Khalifa Haftar - a former general in Gaddafi's army - seized largely-desert southern Libya earlier this year before heading to the coastal capital this month, where they are ensconced on the south side.
The United Nations, United States, European Union and G7 block have all appealed for a ceasefire and a return to UN peace plan, but Haftar has so far not heeded them.
A warplane took out Tripoli's only functioning airport on Monday, and the number of displaced - 3,400 at the last UN count - is mounting alongside the casualties.
The conflict threatens to disrupt oil supplies, boost migration to Europe and scupper hopes for an election to end rivalries between parallel administrations in east and west.
UN officials said they were concerned that civilians could be used as human shields or forcibly recruited to fight.
"The people of Libya have long been caught between numerous warring parties, with some of the most vulnerable suffering some of the gravest violations of their human rights," UN Human Rights High Commissioner Michelle Bachelet said.
"I appeal to all sides to come together to avoid further senseless violence and bloodshed."
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