A boy holds his father's rifle during a rally by followers of the Houthi movement in Yemen's capital Sanaa Aug. 11, 2015. Mohamed al-Sayaghi/Reuters

Children constitute roughly one-third of the fighters in Yemen, according to a United Nations official cited by Gulf News Sunday. The official said a growing number of children have been pulled into the conflict as it has intensified in recent months.

“We are seeing children in battle, at checkpoints and, unfortunately, among [those] killed and injured,” Julien Harneis, Unicef’s representative in Yemen, reportedly said during a stop in Geneva.

Exactly how many children are fighting in Yemen is hard to determine, but officials estimated about 30 percent of the fighters in armed groups were minors. The U.N. documented the recruitment in 2013 of 106 children, some as young as 6 years old, but indicated the comparable number may have risen sharply since then.

Child soldiers were reportedly particularly visible among Houthi factions, as children were spotted guarding buildings and manning checkpoints. At least 77 children have been killed in fighting since the country spiraled into a civil war in March, but Harneis said the conflict-related deaths among children were likely much higher than has been reported.

A child soldier is defined by international law as any person below 18 years old who has been used by armed groups for any purpose, including not only as fighters but also as cooks, sexual partners and spies. There are an estimated 250,000 child soldiers in the world today, according to War Child, an advocacy group that provides support for children in war.

Groups fighting in Yemen -- encompassing the Houthi fighters believed to be backed by Iran and the Saudi-led coalition opposing them -- have repeatedly been accused of committing human-rights violations that some have alleged amount to war crimes. Fighting has intensified as Saudi Arabia and other Arab allies have sought to bolster the Yemeni government, which lost control of much of the country in March. The government has regained its grip over a number of key areas in recent months, but the rebels have maintained their hold over much of the nation.

The U.N. said fighting in Yemen has killed more than 3,640 people since late March, estimating that about one-half of that number were civilians, Hurriyet Daily News reported.