WHO director-general Tedros Adhanom Ghebreyesus said two paramedics were also killed in an attack on another health facility.
"The killings in the last 24 hours of 14 health workers in southern Lebanon mark a tragic development in the escalating Middle East crisis," Tedros said in a post on social media platform X.
Twelve of the medics were killed in a strike on the Bourj Qalouiyeh centre late on Friday, local time, Tedros said.
The other two died in a strike on a health facility in Al Sowana earlier in the day.
Israel has launched an extensive bombing campaign against the powerful Lebanese armed group Hezbollah, which has killed more than 770 people and displaced hundreds of thousands more.
Hezbollah has fired hundreds of rockets across the border.
Earlier, the Israeli military had warned it may strike ambulances and medical facilities which it says are being used unlawfully by Hezbollah.
"As part of its terrorist activities, Hezbollah is using ambulances extensively for military purposes," the Israeli military's Arabic spokesman Avichai Adraee said on X, adding that such use must immediately stop.
"If this practice does not stop, Israel will act in accordance with international law against any military activity carried out by the terrorist group Hezbollah using these facilities and ambulances," Adraee said.
A Hezbollah official said the group was not using ambulances and medical facilities for military purposes.
At least 26 medics and first responders have been killed in Israeli strikes since March 2, according to the Lebanese Health Ministry.
Also on Friday, Israeli aircraft dropped flyers over Beirut threatening to inflict damage on Lebanon similar to the devastation wrought on the Gaza Strip during Israel's two-year war with Palestinian militant group Hamas.
An Israeli official told Reuters the campaign against Hezbollah would likely be intensified and continue even after strikes on Iran die down.
The official said attacks on civilian infrastructure were being debated by the decision-makers.
Hezbollah leader Naim Qassem said his group was prepared for a long confrontation.
Israeli newspaper Haaretz reported on Saturday that Israel and Lebanon were expected to hold direct talks in the coming days, citing two sources with knowledge of the matter.
US President Donald Trump's son-in-law Jared Kushner will be involved in the talks that may be held in Paris or in Cyprus, with Israeli Prime Minister Benjamin Netanyahu's confidant Ron Dermer leading the Israeli delegation, Haaretz said.
The negotiations were expected to focus on ending fighting in Lebanon and disarming Hezbollah, Haaretz said.
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