Airstrikes carried out by a Saudi-led coalition in Yemen that killed dozens in the northern city of Saada were a breach of international laws of war, a rights group says.

“These attacks appear to be serious laws-of-war violations that need to be properly investigated,” Sarah Leah Whitson, Human Rights Watch’s director for the Middle East and North Africa, said Tuesday as the group released its report.

The report investigated a dozen airstrikes between April 6 and May 11 that killed 59 people – 14 women and 35 children.

The release of the report came one day after U.N. Secretary-General Ban Ki-moon deplored airstrikes Sunday on a U.N. compound in Aden that caused “serious damage” to the U.N. Development Program office and injured a guard.

Ban urged a full investigation into the incident and urged that the responsible parties be held accountable. “Ensuring accountability is indispensable in preventing such incidents,” he said.

The U.N. chief also repeated his call for the warring sides to respect international humanitarian law, which requires that belligerents avoid attacks on civilians and civilian facilities, including U.N. staff and premises.

Since the Saudi-led airstrikes began on March 26, similar pleas have been made by other top U.N. officials and the International Committee of the Red Cross.

The Office of the U.N. High Commissioner for Human Rights said that between March 26 and Thursday, at least 1,498 civilians have been killed and 3,312 injured in Yemen fighting. Sixty-two civilians were killed in the week ending Thursday.

The Human Rights Watch report highlights that under the laws of war civilians may never be targeted deliberately.

“Attacks that fail to discriminate between civilians and combatants or that cause civilian harm disproportionate to the expected military gain of an attack are prohibited,” the report said. “Warring parties are required to take precautionary measures to minimize incidental loss of civilian life, injury to civilians, and damage to civilian objects. This includes avoiding the deployment of forces in densely populated areas.”

The report, which drew on 28 interviews with eyewitnesses and relatives of victims as well as analysis of satellite imagery, says Saudi airstrikes hit at least six houses in the city of Saada, in an area of Yemen that is home to the Houthi rebels, whose advance across the country triggered the Saudi bombing campaign.

Human Rights Watch said it found no evidence that the houses were being used for military purposes. The group said Saudi Arabia had not responded to a May 20 letter seeking information on the intended targets of the airstrikes.

“In at least two of these attacks, munitions dropped from coalition aircraft struck the houses more than once, suggesting that they were the intended targets,” the report said.

It also found that coalition aircraft attacked and seriously damaged at least five main markets in Saada.

“As with the residential houses, multiple strikes on some of the markets suggest that they were the intended targets,” the report said. “Markets are normally civilian objects and should not be attacked. Human Rights Watch saw no evidence indicating that the markets had been used for military purposes, such as for the storage of significant munitions.”

Moreover, Human Rights Watch investigators found evidence at the scene of the strikes that U.S.-made bombs were used in the attacks.

At a government building in Saada, Human Rights Watch said, it photographed “the remnants of an MK-83 air-dropped 1,000-pound bomb made in the U.S.”

The United States is not a member of the Saudi-led coalition but has provided logistical support, including aerial refueling for attack aircraft.

The campaign is aimed at blunting the offensive by the Houthis, who practice an offshoot of Shiite Islam and are backed by Iran. The rebels captured the coutry’s capital, Saana, last year and earlier this year drove the Saudi-backed Yemeni president, Abed Rabbo Mansour Hadi, into exile.

The report also concluded that a Saudi military spokesman’s declaration that the entire cities of Saada and Marran were military targets was “a clear violation of the laws of war.” Brig. Gen. Ahmed al Assiri made the declaration May 8.