South Sudan refugee children share a meal at Uganda's newly opened Palabek Refugee Settlement in the northern part of the country, April 13, 2016. (Xinhua/Ronald Ssekandi)

UNITED NATIONS, April 24 (Xinhua) -- More than 25 million children between six and 15 years old, or 22 percent of children in that age group, are out of school in conflict zones across 22 countries, the United Nations Children's Fund (UNICEF) said Monday.

At the primary school level, South Sudan has the highest rate of out-of-school children with close to 72 percent of children missing out on education, followed by Chad and Afghanistan, said Stephane Dujarric, spokesperson for UN Secretary-General Antonio Guterres, at the daily news briefing, citing UNICEF.

The three countries also have the highest rate of girls who are out of school, he said.

At the lower-secondary school level, the highest rates are found in Niger, South Sudan and the Central African Republic, said the spokesperson.