As Yemen enters its third year of civil war, civilians are facing the prospect of another deadly outbreak of cholera, the UN children’s fund has warned.

“Much more attention (needs to be paid to) the situation in Yemen. This has been rightly described as one of the worst humanitarian crises the world has ever known,” Unicef’s regional director for the Middle East and North Africa Geert Cappelaere told reporters in Amman on his return from a week-long trip to the decimated country.

“Let us not fool ourselves. Cholera is going to come back,” he said on Sunday. “In a few weeks from now the rainy season will start again and without a huge and immediate investment, cholera will again hit Yemeni children.”

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Cholera – an acute diarrhoeal infection caused by ingestion of food or water contaminated with the bacterium vibrio cholera – can kill the vulnerable, such as the very old, young and otherwise sick in hours if fluids are not replaced.

More than 11 million children suffered from cholera and diptheria in 2017 thanks to the breakdown of Yemen’s healthcare and sanitation systems and difficulty importing vaccinations, Unicef said.

Unicef was crippled by months of negotiations with both sides for permission for an effective vaccination programme, Mr Cappelaere added.

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In addition to the worst outbreak of the easily-treatable disease in modern history, aid organisations say Yemeni children are also killed or seriously injured at a rate of five a day thanks to fighting between government forces and Houthi rebels, as well as a Saudi-led bombing campaign on opposition areas.

“We are using endless time, energy and money for issues that we should never have to negotiate. The lives of children should not be negotiable,” he said.

“None of the parties in this war have shown for a single second any respect to the sacred principle of the protection of children.”

The appeal from Unicef comes on the eve of the third anniversary of Yemen’s devastating civil war, which has left two thirds of the 28 million strong population dependent on aid to survive and eight million people on the brink of famine.

Since March 2015 Saudi Arabia and its regional partners have conducted an extensive bombing campaign on the Iran-backed Houthis in the capital Sanaa at the request of the exiled, internationally recognised Yemeni President Abdrabbuh Mansour Hadi.

The Saudi economic strangulation, blockades on Yemen’s air and seaports preventing the import of food and medicine and the targeting of vital infrastructure such as roads and bridges – and in some cases civilian buildings such as hospitals, and funeral gatherings – have contributed to the dire situation.

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Western governments including the UK have been heavily criticised for selling weapons export licences to Saudi Arabia, which rights groups say are destined for use in Yemen’s war.

Also on Sunday, debris from a barrage of seven ballistic missiles fired at the Saudi capital Riyadh killed one civilian and injured two others, the first such death in the three-year-old conflict to date.

Rebel rockets are almost always intercepted and deflected by Saudi anti-missiles systems.