(CNN) The war in Yemen is now the world's worst humanitarian crisis, with more than 22 million people -- three-quarters of the population -- in desperate need of aid and protection,United Nations Secretary General Antonio Guterres said.

As the conflict enters its fourth year, millions are without access to clean drinking water and the country is at high risk of a cholera epidemic, Guterre said at a donor conference in Geneva on Tuesday.

He said over 8 million people in the country "did not know where they will obtain their next meal," and that "every ten minutes, a child under five dies of preventable causes."

With many struggling to support their families, child marriage rates have also risen. "Nearly two-thirds of girls are married before the age of 18, and many before they are 15," said Guterres.

More than half of the required funds needed for the UN's humanitarian response plan -- $2.96 billion -- have not been met, Guterres told the conference.