BEIRUT, April 15 - An additional three million children in Syria and Iraq have been forced out of school by conflict, UNICEF says in a new report.

Authored jointly with UNESCO's Institute for Statistics, the report says the rates of out-of-school children in the region had been declining "often by as much as half".

"But in recent years, progress has stalled," it says, with 4.3 million primary-aged children and 2.9 million lower secondary-aged children out of school.

An additional 5.1 million children are not receiving pre-primary school education, bringing the total number of the region's children out of school to 12.3 million.

That figure represents around 15 per cent of the children in the Middle East who should be receiving pre-primary, primary or secondary education.

Yemen had the worst rate of pre-primary school age children receiving an education, with only six per cent in school.

Djibouti and Sudan had the worst rates for secondary school-age children, followed by Iran and Morocco.

The report says a study of nine countries in the region revealed a range of reasons why children were out of school, including poverty.

In many cases, families could not afford costs associated with schooling, including books and uniforms, or the loss of income from a child who could be put to work.

"Children from poor, disadvantaged families are most likely to be excluded from schooling, even though they have the most to gain," Maria Calivis, UNICEF's regional director, said at the report's launch in Beirut on Wednesday.

Gender discrimination also remains a factor.

"Girls are undervalued and, since they are not expected to work, their families see no need for them to learn," the report says, adding that early marriage is also an issue in most countries in the region.

"It's the poor, rural girls who are the most disadvantaged," said UNICEF's Dina Craissati.

Elsewhere, violence is a problem - either inside schools, or in conflict zones like Syria and Iraq, where millions of children no longer have access to education and schools have been caught up in violence.