International rights groups on Monday condemned an attack on a prison in Yemen's besieged city of Taez that left six women and a child dead.

The internationally-recognised government has accused the Iran-aligned Huthi rebels of carrying out Sunday's attack.

The rebels targeted the female section of the prison with mortar shells, according to the government's Saba news agency.

The Huthis "categorically denied" responsibility for the attack, in a tweet by the spokesman for their armed wing, Yahya Saree.

UN High Commissioner for Human Rights Michelle Bachelet said the Taez attack could amount to a war crime.

"We understand three shells, allegedly fired by the Huthis, hit the women's section of the prison," she said in a statement.

"This attack appears to be in breach of international humanitarian law, and depending on the circumstances could amount to a war crime," she said.

Doctors Without Borders (MSF) said its hospital in Taez received the casualties.

"MSF-supported Al-Thawra Hospital in Taez city received the bodies of six women and one child who were killed in an attack on the central prison in Taez," it said on Twitter.

The government said 28 other female prisoners were wounded.

"Taez citizens continue to suffer from the ongoing violence in the sixth year of the protracted conflict in Yemen," MSF said.

"These attacks on civilians, whether indiscriminate or targeted, are unjustifiable breaches of international humanitarian law."

The International Committee of the Red Cross said specifically that attacks on prisons were banned under international law.

"The ICRC deplores yesterday's attack on Taez central prison that left women and children dead and injured," the ICRC said on Twitter.

"Prisons and their inmates are protected under international humanitarian law and can not be a target", it said.

Taez, a city of 600,000 people in southwest Yemen, is under government control but has been under siege by rebels in recent years.

Tens of thousands of Yemenis have been killed in more than five years of fighting that has devastated the impoverished Arab nation.

Yemen's broken healthcare system has so far recorded no case of the COVID-19 illness, but aid groups have warned that when it does hit, the impact will be catastrophic.

The country is already gripped by what the UN calls the world's worst humanitarian crisis.