Yemen's Houthi rebels are recruiting fighters as young as 15, including by using religious schools to lure teenagers into their ranks without their parents' knowledge, according to a rights group.

Amnesty International described the "appalling" practices in a report released on Tuesday, citing family members of four boys - aged 15 and 17 - who were recruited by the rebels.

"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 International's Beirut regional office.

"This is a shameful and outrageous violation of international law," Hadid said, calling on the rebels to halt such recruitment and release the child soldiers.

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One family member whose 16-year-old brother was taken told Amnesty that the young soldiers were "just excited to shoot Kalashnikovs and guns and wear military uniforms.

"They [the Houthis] have been saying that there are so few fighters [at the front line], they are going around taking one [recruit] from each family," said the anonymous interviewee.

"If the son dies at the front line, a monthly salary and a gun are given to the father to keep them quiet."

The report by the UK-based group also said that many families feared reprisals against their children who had been taken by the Houthis, or against other children or family members if they dared speak out about their recruitment.

"Many children [are recruited] but people don't dare talk or follow up," said one father, according to the report.

"They are afraid of being detained."

1,500 child soldiers

Also on Tuesday, Ravina Shamdasani, a UN spokeswoman, said that about 1,500 boys were verified as child soldiers in Yemen, but the actual number of children who have been drawn into the war was probably far higher.

Shamdasani said at a news conference in the Swiss city of Geneva that most of the boys had been recruited by Houthis over the past three years.

She said the recruiters often entice the young recruits by promising them financial rewards or social status.

"Many are then quickly sent to the front lines of the conflict or tasked with manning checkpoints," Shamdasani said.

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The UN human rights office demanded an immediate release of all child soldiers in the country.

The conflict in Yemen pits the rebels, who control the capital and much of the country's north, against a Saudi-led coalition fighting to restore the internationally recognised government.

According to UN figures, the nearly two-year conflict has killed at least 10,000 people and wounded 40,000.