WHO's emergency response director, Peter Salama, during a press briefing in the Swiss city of Geneva on Friday, called for a three-day truce to allow vaccinations.
"We have had two major waves of cholera epidemics in recent years and unfortunately the trend that we have seen in recent days to weeks suggests that we may be on the cusp of the third major wave," the official said,
"We are calling on all parties to the conflict to act in accordance with international humanitarian law and to respect the request of the international community for three full days of tranquility to lay down arms and to allow us to vaccinate the civilian population," he added.
The warning came a day after a deadly airstrike by Saudi Arabia on Yemen's largest hospital in Hudaydah, where WHO staff were preparing the cholera vaccination drive, Presstv reported.
Elsewhere in his remarks, Salama explained that the vaccination drive is set to run from Saturday to Monday and is aiming to target more than 500,000 people.
Earlier on Friday, Lise Grande, the UN's humanitarian coordinator in Yemen, voiced shock at the strike on the al-Thawra hospital, saying it had put the cholera response at risk.
"This is shocking," Grande said, adding, "Hospitals are protected under international humanitarian law. Nothing can justify this loss of life."
Dozens were killed after warplanes of the Saudi-led military coalition hit a hospital and a fishing harbor in Hudaydah.
Meanwhile, UNICEF Executive Director Henrietta Fore also said in a statement the airstrikes earlier this week damaged a sanitation facility and a station that supplies most of the city's water. "Attacks on water infrastructure jeopardize efforts to prevent another outbreak of cholera and acute watery diarrhea in Yemen."
WHO said in July that the cholera outbreak in war-torn Yemen had killed 1,500 people since late April.
Cholera infection first became epidemic in Yemen October 2016 and spread until December the same year, when it dwindled. The second outbreak began in the Arabian Peninsula country in April last year.
UN set to convene Yemen talks in Geneva next month
In another development, UN envoy to Yemen Martin Griffiths told the Security Council on Thursday that the world body will invite warring sides in Yemen for talks on September 6 in Geneva to discuss a framework for peace negotiations.
Griffiths said that "a political solution" to end the war in Yemen was "available". He also urged world powers to support the new push for peace negotiations.
"These consultations will provide the opportunity for the parties, among other things, to discuss the framework for negotiations, relevant confidence-building measures and specific plans for moving the process forward," said Griffiths.
The UN envoy also said that he was "still trying" to negotiate a deal to avoid a full-blown battle for Hudaydah, which is a key entry point for humanitarian aid.
UN-brokered political talks on Yemen broke down in 2016.
The Yemeni Ministry of Human Rights announced in a statement on March 25 that the war had left 600,000 civilians dead and injured until then. The war and the accompanying blockade have also caused famine across Yemen.
The Saudi-led aggression has also taken a heavy toll on the country's infrastructure, destroying many hospitals, schools, and factories. The United Nations has already said that a record 22.2 million Yemenis are in need of food aid, including 8.4 million threatened by severe hunger.
Several Western countries, the United States and Britain in particular, are also accused of being complicit in the ongoing aggression as they supply the Riyadh regime with advanced weapons and military equipment as well as logistical and intelligence assistance.