By Kareem Fahim and C.J. Chivers, New York Times

CAIRO — A Saudi Arabia-led military coalition used bombs supplied by the United States in an attack on a market in Yemen last month that killed at least 97 civilians, including 25 children, Human Rights Watch said in a report released Wednesday.

The group said it had found fragments of two U.S.-made bombs at the market, in the northern district of Mastaba, linking the United States for the first time to the March 15 airstrikes, which were believed to be the deadliest coalition bombings during Yemen’s yearlong civil war. The high death toll, along with images of children killed in the blasts, ignited international outrage and prompted calls for an investigation.

The Saudi-led coalition has been criticized for carrying out indiscriminate airstrikes that have hit markets, hospitals and homes as it has waged war against the Houthis, a rebel group from Yemen’s north that seized power from the government last year.

Coalition airstrikes have caused most of the civilian deaths in the conflict, according to the United Nations, and have led to mounting calls in Europe for an arms embargo on Saudi Arabia. An airstrike on another market, in February near Sanaa, the capital, led U.N. Secretary-General Ban Ki-moon to call for an inquiry.

The debate in the United States over the airstrikes has been much more muted, in part because the Obama administration has provided few details about its role in the air campaign. U.S. officials have said they provide assistance to the coalition, including intelligence from reconnaissance drones, airborne fuel tankers and advanced munitions.

The assistance is coordinated by a 45-person U.S. military planning group with personnel in Saudi Arabia, Bahrain and the United Arab Emirates, according to U.S. officials.

In Houthi-controlled northern Yemen, where the airstrikes have been concentrated, the Obama administration’s participation in the war has fueled growing anger at the United States, residents said. In its report, Human Rights Watch said the United States might be jointly responsible for war crimes violations if it participated “in specific military operations, such as providing advice on targeting decisions and aerial refueling during bombing raids.”

“The U.S. is obligated to investigate allegedly unlawful attacks in which it took part,” the group said.

In response to questions about the Human Rights Watch report, Col. Patrick Ryder, a spokesman for the U.S. Central Command, or CENTCOM, wrote in an email that the “decisions on the conduct of operations to include selection and final vetting of targets in the campaign are made by the members of the Saudi-led coalition, not the United States.”

“The U.S. is confident that the information that we relay and noncombat support we provide to Saudi Arabia and other coalition members is sound and provides them the best options for military success consistent with international norms and specifically mitigating the potential for civilian casualties,” he added.

“We have consistently reinforced to coalition members the imperative of target analysis and precise application of weapons in order to identify and avoid structures and areas that, if struck, could result in civilian casualties.”

Human Rights Watch said its researchers had found fragments of what it said was a 2,000-pound U.S. bomb called the MK-84 during a visit to the market March 28.

The group reviewed photographs and footage showing fragments from a second bomb, found by journalists from ITV, the British television network, and determined that it was also an MK-84. The size of the ordnance was determined in part by reviewing photographs of bomb craters, the group said.

Establishing the precise size of an air-delivered bomb is hard to do by crater analysis, and it was impossible to independently verify the organization’s claims. But if confirmed, the use of 2,000-pound bombs would reflect a decision by the Saudi-led coalition that carried substantial risks for civilians.

The 2,000-pound general-purpose bomb, of the U.S. standard Mark 80 series, is the largest of its class. U.S. warplanes typically carry smaller bombs, often in the 500-pound class, in part to reduce property damage and dangers to noncombatants.

A spokesman for the Saudi coalition did not immediately return calls seeking comment on the report. The spokesman, Brig. Gen. Ahmed al-Assiri, previously told Reuters that the coalition struck the market after acting on information provided by anti-Houthi forces loyal to Yemen’s exiled government.

Witnesses told Human Rights Watch that the bombs fell about noon, five minutes apart. One landed near a tomato seller and the other near the entrance to the market.

A witness told the group that he saw the bodies of 10 Houthi fighters among the dead and that some Houthis frequented a restaurant about 200 feet from where one of the bombs fell.

Mohamed Bikili, who had gone to the market that day to buy food, was among the victims, according to his father, Mansoor Ali Bikili. The father said he headed toward the market after hearing the first airstrike and, when he arrived, after the second bombing, “the dust in the market had turned black.”

Mohamed Bikili, 18, was nowhere to be found. Over the next few days, Bikili recovered parts of what he believed to be his son’s body, strewed across the market, he said.

Mark Mazzetti contributed reporting from Washington, and Shuaib Almosawa from Sanaa, Yemen.

To Learn More:

               Yemen: US Bombs Used in Deadliest Market Strike (Human Rights Watch)

U.S. Cluster Bombs Used by Saudis in Yemen May Violate U.S. Law (by Rick Gladstone, New York Times)

U.S. Plans to Replenish Saudi Missiles Used in Air Strikes on Yemen that U.N. Says May Constitute Crimes against Humanity (by Michelle Nichols, Reuters)

Red Cross Accuses U.S.-Backed Saudis of Deliberately Attacking Health Care Facilities in Yemen (by Noel Brinkerhoff, AllGov)

U.S. “Concern” Over Yemen Crisis Belies Its Military Support of Saudi Coalition Bombing (by Noel Brinkerhoff, AllGov)

In an Overlooked War, Saudis Use U.S.-Made Weapons to Kill Civilians in Yemen (by Noel Brinkerhoff, AllGov)