Yemen is likely to be struck by another "major wave" of cholera cases, the World Health Organization said Friday, as it called for a three-day truce to allow vaccinations.

The warning came a day after a deadly airstrike on the country's largest hospital in the rebel-held port city of Hodeida, where WHO staff were preparing the cholera vaccination drive at the time of the attack.

"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," WHO's emergency response director, Peter Salama, told reporters in Geneva.

"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.

Yemen, described by the UN as the world's worst humanitarian crisis, has seen more than a million suspected cholera cases amid the conflict between Iran-backed Huthi rebels and government forces supported by a Saudi-led coalition.

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, the UN's humanitarian coordinator in Yemen, Lise Grande, voiced shock at the strike on the Al-Thawra hospital, saying it had put the cholera response at risk.

Yemen's war has killed nearly 10,000 people, with more than eight million Yemenis at risk of starvation.