Saudi-led coalition air strikes on a rebel stronghold in Yemen have destroyed houses, markets and a school, killing dozens of people in what could amount to war crimes, Human Rights Watch said Tuesday.

The US-based rights group said it had investigated the attacks on the northern city of Saada and found that at least 59 people had died in a month, from April 6 to May 11, including 14 women and 35 children.

Satellite imagery showed over 210 strike sites in the city, a stronghold of the Shiite Huthi rebels, damaging or destroying hundreds of buildings, said the report.

Six houses were hit in Saada, as were an empty school, a cultural center, five markets and a petrol station crowded with motorists, according to the report.

In one of the deadliest incidents, a bombing raid on May 6 killed 27 members of one family including 17 children.

Attacks on houses alone have killed 51 people, all of them civilians, according to Human Rights Watch, which dispatched two researchers to the city last month to interview witnesses.

Belkis Wille, one of the researchers, said "Yemen has been absolutely brought to its knees in this war."

Once a city of about 50,000, Saada has been turned into a "ghost town" as a result of the air war which has seen northern Yemen hit hardest by the bombings, she told a news conference.

Coalition authorities last month declared all of Saada to be a military target and warned its residents to leave.

"You cannot decide that an entire area is fair game," said HRW's crisis advocacy director Philippe Bolopion.

Civilians many not be able to leave, due to fuel shortages, he added.

"Not only were these attacks unlawful because of the apparent absence of any military target, but they contributed to civilian hardship in the city, where people are suffering from shortages of food, water, and fuel," said the report.

- Saada a military target -

The nine-nation Saudi-led coalition launched air strikes on Yemen on March 26 to halt an advance by the Iran-backed Huthis who drove the president into exile.

Saudi Arabia and its Gulf allies are demanding that the Huthis pull back from territory seized in their offensive and that President Abedrabbo Mansour Hadi be restored to power.

A week of UN-brokered talks in Geneva earlier this month failed to narrow differences.

Human Rights Watch called on the coalition to investigate the attacks to determine whether they constitute a violation of international humanitarian law.

The rights group cautioned that it had not found evidence that the coalition deliberately targeted civilians, but said the coalition's declaration that all of Saada was a target was illegal.

Andreas Krieg, a consultant to the Qatar armed forces and a lecturer at King's College in London, said the coalition was seeking to avoid civilian casualties.

Many civilians have been killed because of the terrain which includes inaccessible valleys and cities, said Krieg.

He stressed that the Huthis "use civilians as human shields" by hiding among them.

HRW researchers who traveled to Saada reported that they saw Huthi vehicles mounted with anti-aircraft weapons.

UN Secretary-General Ban Ki-moon has demanded a full investigation after Saudi coalition air strikes hit a UN compound in the southern city of Aden on Sunday, injuring a guard and seriously damaging the building.