ABS, Yemen -- The taxi driver happened by just after a volley of airstrikes hit a highway in western Yemen. The driver, Mohammed al-Khal, stopped, took a wounded ice cream vendor into his car and rushed him to the nearest hospital.

But the warplanes were still hunting. Moments after al-Khal pulled up at the hospital in the town of Abs, hell was unleashed.

A missile struck just outside the hospital entrance, “like a ball of fire,” one witness said. Al-Khal, a father of eight, was incinerated in his car. The blast ripped through patients and family waiting in an outdoor reception area. Nineteen people were killed, along with two civilians killed on the highway.

Get Breaking News Delivered to Your Inbox

The attack in August typified what has been a pattern in the nearly 2-year-old air campaign by Saudi Arabia and its allies against Yemen’s Shiite rebels, known as Houthis. Rights groups and U.N. officials say the U.S.-backed coalition has often either deliberately or recklessly depended on faulty intelligence, failed to distinguish between civilian and military targets and disregarded the likelihood of civilian casualties.

Experts say some of the strikes amount to war crimes.

“The Saudis have been committing war crimes in Yemen,” said Gabor Rona, a professor teaching the laws of war at Columbia University. He pointed to “indiscriminate targeting, that is, attacks in which the attacker makes no effort to distinguish between combatants and civilians.” And he warned that American personnel helping the coalition “may also be guilty of war crimes.”

Nearly 4,000 civilians have been killed since early 2015 and an estimated 60 percent of them died in airstrikes by the Saudi-led coalition, the U.N. says. Around one in three strikes hit civilian targets, according to the Yemen Data Project, an independent group of researchers that has put together a database of more than 8,000 strikes.

The strikes in Abs were notable: The original target was a small checkpoint manned by two rebel fighters on a highway far from any frontline. Three missiles were fired on the highway and one at the hospital. Of 21 people killed, none was a combatant. Moreover, the hospital itself was on the coalition’s own computerized list of sites that should not be targeted.

Each strike that day was carried out with a Paveway guided missile system, built by an American company and sold to Saudi Arabia - a sign of how the United States has become mired in Yemen’s war. Washington and its allies have sold billions of dollars in weapons to Saudi Arabia for the campaign, and the U.S. military has been providing it with intelligence, satellite imagery and logistical help.

Washington underlines that it does not make decisions on strikes, and it calls on the coalition to investigate any claims of violations. “U.S. security cooperation with Saudi Arabia is not a blank check,” National Security Council spokesman Ned Price said in October.

The coalition denies neglect, saying it does its utmost to avoid civilian casualties and noting the rebels often operate among civilians.

“This is the fog of war,” the coalition’s spokesman, Saudi Gen. Ahmed al-Asiri, told The Associated Press when asked if there is a pattern of civilian deaths.

“In war, one plus one doesn’t equal two. In war, there are many changes taking place around the clock. In war, there are decisions that should be taken fast,” he said from the Saudi capital, Riyadh.

But critics say the American and international backing and lack of independent investigation have given Saudi Arabia and its allies a free rein.

“We believe that the coalition understood that ... it has a green light to commit more massacres in Yemen,” said Abdel-Rashed al-Faqeh, the head of Muwatana, one of Yemen’s most prominent rights groups.

Saudi Arabia launched the coalition campaign in a bid to restore the internationally recognized government of President Abed Rabbo Mansour Hadi, after the Houthis overran the capital Sanaa and the north of the country. The Houthis are allied with troops loyal to Hadi’s predecessor Ali Abdullah Saleh, who was ousted after a 2011 pro-democracy uprising.

Saudi Arabia, which calls the rebels a proxy of its regional rival, Iran, has backed an array of military units and fighters against the Houthis.

A Yemeni man inspects the damage following a bomb explosion at the Badr mosque in southern Sanaa on March 20, 2015. Getty Images

The war has been devastating for the country of 24 million. Around 3 million have been driven from their homes, fleeing to other parts of Yemen. The bombardment, fighting and a coalition blockade have fueled widespread hunger.

Human Rights Watch, Amnesty International and the U.N. human rights office have also reported possible war crimes by the Houthi rebels, citing their shelling of civilian areas and basing their fighters in schools and other civilian locations.

But the scope of the air campaign has brought widespread destruction. Warplanes have hit medical centers, hospitals, schools, factories, infrastructure and roads, as well as markets, weddings and residential compounds. The Yemen Data Project, for example, documented nearly 60 strikes on medical facilities, most of them in the Houthi heartland in the north, though it says it does not track casualty figures because of how difficult it is to verify events on the ground.

The coalition, which says it investigates claims of civilian casualties, has made nine investigations public. In most cases it said the strikes were against a justified military target.