iStock/Thinkstock(NEW YORK) — A simmering conflict in South Sudan has forced more than one million children to seek refuge in neighboring countries, creating one of the most significant refugee crises in the world, the United Nations said on Sunday.

“No refugee crisis today worries me more than South Sudan,” said Valentin Tapsoba, Africa Bureau director at UNHCR, the U.N. Refugee Agency. “That refugee children are becoming the defining face of this emergency is incredibly troubling. We, all in the humanitarian community, need most urgent, committed and sustainable support to be able to save their lives.”

The U.N. estimates that some 1.8 million people have fled South Sudan, more than 6 in 10 of whom are minors under 18 years of age.

At least 75,000 minors have crossed South Sudan’s borders either unaccompanied or separated from their families.

More than 1,000 children have been killed since conflict flared up in 2013, and more than three quarters of the country’s children are not in school, the highest share of any country in the world, the U.N. said.

Some five million people in South Sudan struggle to find enough food, according to the U.N. World Food Programme, with more than 250,000 children facing malnutrition as a result.

South Sudan became the world’s newest country when it seceded from Sudan in 2011 after nearly four decades of civil war that crippled development and left some 2.5 million people dead.

In 2013, the young country again fell into conflict with the two main warring parties allied along ethnic lines. Armed groups on both sides have been accused of recruiting children and regularly targeting civilians with war crimes such as sexual assaults, ethnically motivated killings and attacks on aid workers.

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