"More than eight million children -- nearly half of the 17 million of school age -- have gone approximately 484 days without setting foot in a classroom," the children's rights organisation said in a statement.

Sudan has been ravaged since April 2023 by a power struggle between the army and the paramilitary Rapid Support Forces (RSF).

This is "one of the longest school closures in the world," the British NGO said.

"Many schools are closed, others have been damaged by the conflict, or are being used as shelters" for the more than seven million displaced people across the country, it added.

North Darfur in western Sudan is the country's hardest-hit state: only three percent of its more than 1,100 schools are still functioning.

In October the RSF seized the city of El-Fasher, the capital of North Darfur and the last of Darfur's five capitals that had remained outside their control.

West Darfur, West Kordofan and South Darfur follow with 27 percent, 15 percent and 13 percent of their schools operating respectively, according to the statement.

The NGO added that many teachers in Sudanese schools were leaving their jobs due to unpaid salaries.

"We risk condemning an entire generation to a future defined by conflict," without urgent investment, said the NGO's chief executive Inger Ashing.

The conflict, which has claimed tens of thousands of lives, has triggered the "world's worst humanitarian crisis", according to the UN.

On Sunday, UN Human Rights commissioner Volker Turk condemned the increasing number of attacks against "essential civilian infrastructure" in Sudan, including hospitals, markets, and schools.

He also expressed alarm at "the arming of civilians and the recruitment of children."

The UN has repeatedly expressed concern about the "lost generation" in Sudan.