The United Nations children's agency says attacks on schools in Afghanistan tripled in 2018, compared to the year before.

UNICEF's press release on Tuesday says the number of attacks on schools went from 68 in 2017, to 192 in 2018. It was the first time school attacks had increased since 2015.

The agency says one of the reasons for the spike was that schools were used as voter registration and polling centers in last year parliamentary elections.

UNICEF also says Afghanistan's conflict and deteriorating security situation left more than 1,000 schools closed last year. Half a million children were denied their right to education.

An estimated 3.7 million children between 7 and 17 years of age — nearly half of all school-aged children — are out of school in Afghanistan.