Two million children are out of school in war-torn Yemen, a fourth of whom have dropped out since the conflict escalated in March 2015, the UN children's agency said Wednesday, WION reports.

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, a 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 says.

"Children out of school face increased risks of all forms of exploitation including being forced to join the fighting, child labour 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," Nyanti added.

Tens of thousands of people, mostly civilians, have been killed since Saudi Arabia and its allies intervened in March 2015 in support of the beleaguered government after the Iran-aligned Huthi rebels captured the capital Sanaa.

The fighting 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.