(CNN) Nearly one in five children live in areas affected by armed conflict and war, according to a new report published by the charity Save the Children.

Some 420 million children were living in conflict-affected areas in 2017, marking the largest number of children living in areas affected by armed conflict than at any time in the past 20 years, it said.

The report defined affected children as those living within 50 kilometers (about 30 miles) of where one or more conflict events took place in a year within a country's borders.

Children were hit hardest by conflict in Afghanistan, Yemen, South Sudan, the Central African Republic, the Democratic Republic of Congo, Syria, Iraq, Mali, Nigeria and Somalia in 2017, according to the report, "Stop the War on Children: Protecting Children in 21st Century Conflict," with research carried out by the Peace Research Institute Oslo.

The report said one reason 2017 saw an increase in children living in conflict-affected areas is that modern warfare has changed the lines of engagement, with fighting in urban areas among civilian populations more common and international rules disregarded.

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