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The number of children who do not attend school is rising, child marriage has not dropped in decades and millions of young children will die mostly preventable deaths by 2030 if global poverty is not addressed, UNICEF said in a bleak report issued on Tuesday.

Poor children are twice as likely as rich children to die before age 5, and poor girls are more than twice as likely to become child brides in signs of troubling inequality, said the annual report by the United Nations' children's agency.

Noting some progress in halving global mortality rates for children under 5 since 1990 and boys and girls attending primary school in equal numbers in 129 countries, the report said such developments have been neither even nor fair, with repercussions for global turmoil.

"Some of the big challenges that we now face, like refugees and migrants, are connected with inequality and poverty," Justin Forsyth, Undine's deputy executive director, told the Thomson Reuters Foundation.

Narrowing that inequity "is good for those children, but it's also good to stop future crises," he said.

The report called for stronger efforts to educate the world's children, noting that on average each additional year of education a child receives increases her or his adult earnings by about 10 percent.

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Also it said for each additional year of schooling completed on average by young adults, a nation's poverty rate drops 9 percent.

About 124 million children do not go to primary and lower-secondary school, a number that has increased about 2 million since 2011, it said.

Children born to educated mothers are almost 3 times less likely to die and more likely to go to school, delay marriage and postpone child bearing, said the report, entitled "State of the World's Children."

The rate of child marriages among the world's poorest girls has remained unchanged since about 1990, and 15 million girls are married as children every year, it said.

If nothing is done, it said 69 million children will die before age 5 from mostly preventable causes by 2030, and nearly of half of them will be from sub-Saharan Africa.

Nine out of ten children in the same region will be living in extreme poverty, which means living on less than $1.90 U.S. per day, it said.

Battling such poverty and equality and promoting education for children, particularly girls, were among the U.n.' s Sustainable Development Goals, a set of 17 goals adopted last fall to tackle an array of global woes by 2030.

Educating children is particularly critical given the global conflicts fueled by decimalization, Forsyth added.

"There is a direct link between children for many, many years missing out on education and then the ability of more extremist elements to organize," he said.

This article was reported for the Thomson Reuters Foundation, the charitable arm of Thomson Reuters, that covers humanitarian news, women's rights, trafficking, property rights and climate change.