Nearly 1,500 children have been recruited by Yemen's warring parties, mostly the Shia Houthi rebels, since March 2015, the United Nations said on Tuesday.
The UN has verified the recruitment of 1,476 children, all boys, between March 26, 2015 and January 31, 2017, said a statement by the spokesperson for the UN High Commissioner for Human Rights, Ravina Shamdasani.
"The numbers are likely to be much higher as most families are not willing to talk about the recruitment of their children, for fear of reprisals," she said.
"Just last week, we received new reports of children who were recruited without the knowledge of their families," she said, adding that children under 18 are either being "misled or attracted by promises of financial rewards or social status".
The Iran-backed Houthis and their allies overran the capital Sanaa in 2014 before sweeping south, forcing the internationally recognised government to flee.
The war escalated in March 2015 when a Saudi-led Arab coalition began a military campaign against the rebels in support of President Abedrabbo Mansour Hadi.
"We remind all parties to the conflict that the recruitment and use of children in armed conflict is strictly forbidden by international human rights law and international humanitarian law," said Shamdasani.
The recruitment of children under 15 "may amount to a war crime," she added.
"We urge them to immediately release such children."
Meanwhile, rights group Amnesty International accused the Houthis of "actively recruiting boys as young as 15".
It cited witnesses speaking of financial incentives to families, including monthly salaries ranging between $80 and $120 for every family of a child "martyr".
"It is appalling that Houthi forces are taking children away from their parents and their homes, stripping them of their childhood to put them in the line of fire where they could die," said Samah Hadid, Deputy Director at Amnesty's Beirut regional office.
Amnesty documented the cases of four boys taken in mid-February, saying that their families later received news that their sons were at an unnamed location on the border with Saudi Arabia.
Shamdasani said 4,667 civilians had been killed in the conflict since March 2015, with 8,180 wounded.
In total, the UN says that more than 7,500 people have been killed in the war and 40,000 wounded.