The education of a further 3.7 million children is at risk as teachers' salaries have not been paid in more than two years, UNICEF said in a statement.  

"Violence, displacement and attacks on schools are preventing many children from accessing school," said Sara Beysolow Nyanti, UNICEF representative in Yemen.

One in five schools in the country can no longer be used as a direct result of the conflict that has devastated Yemen's already fragile education system, the UN agency said, AFP reported.

"Children out of school face increased risks of all forms of exploitation including being forced to join the fighting, child labor and early marriage," Nyanti said.

"They lose the opportunity to develop and grow in a caring and stimulating environment, ultimately becoming trapped in a life of poverty and hardship."

Tens of thousands of people, mostly civilians, have been killed since Saudi Arabia and its allies launched an aggression on Yemen in March 2015 in support of fugitive former president Abd Rabbuh Mansour Hadi, a close ally of Riyadh.

The aggression has displaced millions and left 24.1 million -- more than two-thirds of the population -- in need of aid.

 According to UNICEF, 1.8 million children under the age of five are suffering from severe malnutrition.

 The United Nations has described Yemen as the world's worst humanitarian crisis.