Children as young as eight have been raped in a besieged city in war-hit Yemen, Amnesty International said on Monday, with militiamen among the suspected perpetrators.

Amnesty said it had documented evidence of the rape of three boys and the sexual assault of a fourth in the southwestern city of Taez, controlled by pro-government forces and surrounded by Yemeni rebels.

The families of the four boys, aged eight to 16, told Amnesty their sons had been assaulted over the past eight months, including at a mosque, but authorities in the area had not been responsive.

All four families reported the assaults to the Criminal Investigations Unit in Taez, Amnesty said. No legal measures had been taken.

“Rape and sexual assault committed in the context of an armed conflict are war crimes,” said Heba Morayef, Amnesty's Middle East and North Africa director. “Commanders who fail to stop such heinous acts can themselves be responsible for war crimes.”

The mother of an eight-year-old boy told Amnesty her son had been raped at least twice in 2018 at a mosque, each time by two men including the son of an Islah-affiliated imam.

Al-Islah is a Yemeni extremist party with historic ties to the Muslim Brotherhood and Saudi Arabia.

Medical reports reviewed by Amnesty showed the boy suffered impaired mobility and concussions as a result of assault.

The father of a 13-year-old boy reported his son had been raped by the same two men at the same mosque.

Another boy, who reported being raped in December, told Amnesty he had been assaulted at gunpoint by an “Islahi-aligned militiaman”.

“He was unable to sit afterwards or go to the bathroom for three days,” the mother of the 16-year-old said. “He just sat there staring into space.”

Other unreported cases are likely, Amnesty said, as families are often too afraid of militias, which have flourished in the chaos of war, to come forward.

Convicted sexual violence offenders can be given the death penalty under Yemeni law. A number of men have been publicly executed in recent years for the rape and murder of children.

The war between Yemen's Houthis and a pro-government military alliance led by Saudi Arabia has killed nearly 10,000 people, including 2,200 children, according to the World Health Organization.

Other groups say the toll is far higher.

The conflict has triggered what the United Nations calls the world's worst humanitarian crisis, with nearly 14 million Yemenis at risk of famine due to violence, poverty, disease and blockades.

Both parties stand accused of acts that could amount to war crimes.