Some 1,266 people have also been wounded since the beginning of the battle between Haftar's Libyan National Army (LNA) and forces loyal to the government earlier this month, Tariq Jarasevic, WHO spokesperson, told reporters on Tuesday.
The clashes, which have raised fears of worsening an already-bad conflict in Libya, began after Haftar, ordered his forces to seize Tripoli from the UN-recognised rival Government of National Accord (GNA) on April 4.
Although Haftar has so far failed to wrest the capital from pro-GNA forces, sporadic fighting on the city's outskirts has left casualties on both sides.
According to Jarasevic, at least 21 of the deceased and 69 of the injured were civilians.
He also said WHO has been helping the treatment of the wounded and have assigned a team to the region.
Tripoli's health ministry said it is also trying to help civilians stuck in and around the fighting areas, Al Jazeera's Mahmoud Abdelwahed reported.
Meanwhile, the United Nations High Commissioner for Refugees (UNHCR) said 32,000 civilians have been displaced since the beginning of the conflict.
"We call on the parties for the protection of civilians and humanitarian aid," said UNHCR spokesman Barbar Baloch.
On Monday, the UN's deputy envoy to Libya Maria do Valle Ribeiro expressed concern over "the increasing rate of displacement" and stressed the need for the UN to ramp up its humanitarian response.
'Relatively quite'
The GNA forces announced earlier this week the launch of a new phase of attack to repel Haftar's fighters and recapture the inactive Tripoli international airport, south of the capital.
Abdelwahed, reporting from Tripoli, said: "It's relatively quiet now on the front lines today, but both rival factions on the ground continue receiving military units and detachments from several cities."
"Government forces say that they have managed to cut one of the major supporting lines to Haftar's forces, the highway connecting Tripoli to Gharyan, the central command for the LNA," he said.
"They [GNA] also claim to have almost completed what they call the defence cordon around Tripoli to push Haftar's forces back beyond the administrative borders of the capital city."
Beset by turmoil
LNA spokesman Ahmed Mismari on Monday denied there had been a retreat but said an advance by his forces had slowed because of the dense population in the areas where fighting was taking place.
WHO, the UN's health agency, called for "a temporary cessation of hostilities" and urged "all parties to respect international humanitarian law".
Since launching his push, Haftar's forces have captured the districts of Gharyan and Qasr Bani Ghashir along with several smaller towns. They also seized the capital's shuttered old airport.
Libya has remained beset by turmoil since long-serving leader Muammar Gaddafi was overthrown and killed in a bloody NATO-backed uprising in 2011.
Since then, the country has seen the emergence of two rival seats of power: one in eastern Libya, to which Haftar is affiliated, and another in Tripoli, which enjoys UN recognition.