A bipartisan group of senators on Wednesday urged the Biden administration to do more to ensure that U.S. military support to Saudi Arabia and the United Arab Emirates does not contribute to civilian harm in Yemen, following an internal watchdog report that said the United States has failed to assess how its aid is tied to such casualties.

The report, released publicly in June, after The New York Times disclosed its existence, found that while the Pentagon oversaw $54.6 billion of military aid to Saudi Arabia and the United Arab Emirates from 2015 to 2021, top security officials failed to collect sufficient data and evidence on civilian casualties or monitor the use of American-made weapons.

In a pair of letters to the State Department and Pentagon, Senators Elizabeth Warren, Democrat of Massachusetts, Bernie Sanders, independent of Vermont, and Mike Lee, Republican of Utah, called the administration’s inaction to determine the extent to which U.S. military support has led to civilian harm in Yemen “an unacceptable failure.”
“We urge you to review whether or not the Saudi and Emirati governments are taking the necessary precautions to prevent harm to civilians in Yemen,” the senators wrote. “If either are found to be in violation, we urge State to halt all arms sales to either country until it can verify they are taking steps to protect civilians.”

Civilian casualties have become something of a hallmark of the war in Yemen. For nearly a decade, the Saudi-led coalition that is fighting Houthi rebels for control of Yemen has carried out deadly strikes using American-made combat jets and munitions supplied with the approval of the U.S. government.

In the early days of the war, Saudi jets dropped American-made bombs on a funeral in Yemen’s capital, killing more than 140 people, and a Yemeni school bus, killing 44 boys on a field trip. More than 150,000 people have been killed in the war, including nearly 15,000 civilians, according to an estimate by the Armed Conflict Location and Event Data Project.

On Jan. 21, an airstrike on a prison run by the Houthis killed at least 70 people and injured dozens of others, according to Houthi officials and international aid groups. But deaths have dropped since the warring groups agreed in April to a tentative truce that the United Nations helped negotiate. The truce was extended for two months in early August. U.S. officials said Mr. Biden’s recent visit to Saudi Arabia was partly aimed at trying to bring the war to a permanent end.

The internal report also found that the Pentagon revealed that it does not track how countries have used at least $319 million in logistical support to the Saudis and Emiratis, “meaning civilian harm could be the direct result of aid provided by the United States without our knowledge.”

Lt. Col. Rob Lodewick, a Pentagon spokesman, said in a statement that the Defense Department “remains deeply concerned by all reports of civilian casualties, to include those in Yemen, and will take all available measures to avoid such tragedies.”

He added that the Pentagon “has long since ended all U.S. support for offensive military operations in Yemen by the Saudi-led coalition” and that American officials “consistently emphasize the necessity to uphold the law of armed conflict and prevent civilian harm.”

A spokesman for the State Department did not immediately respond to a request for comment.

In February 2021, Mr. Biden announced the United States would end support for Saudi-led offensive operations in Yemen. However, he said the United States would continue providing “defensive” aid to Saudi Arabia, without saying how his administration would ensure the Saudis did not use that for offensive operations. Saudi officials have long said their actions are to defend against Houthi and Iranian aggression.

The United States continues to sell arms to Saudi Arabia and help maintain the country’s American-made fighter jets and other military equipment.

Bipartisan outrage about the billions of dollars’ worth of munitions the United States provides to Saudi Arabia intensified on Capitol Hill during the Trump administration, after the 2018 killing of Jamal Khashoggi, the Washington Post columnist. U.S. intelligence officials concluded he was murdered by a Saudi hit team directed by Crown Prince Mohammed bin Salman, and lawmakers in both parties moved to block the weapons sales, though President Donald J. Trump circumvented Congress and went through with them anyway.

But the bipartisan anger waned as time went on, particularly after Mr. Biden took office and pledged to end U.S. support for the Saudi-led war in Yemen, including some arms sales.

The letter on Wednesday suggested that the concerns on Capitol Hill have resurfaced, as Mr. Biden seeks to rebuild ties with the kingdom and Prince Mohammed following Russia’s invasion of Ukraine, which affected oil prices, and amid mounting fears about Iran’s nuclear program.

Concerns about how the U.S. government attempts to minimize civilian casualties have mounted in Congress amid growing evidence of recurring episodes over multiple administrations in which civilian bystanders have been killed during drone strikes.

Separate investigations, relying on the military’s own confidential assessments of more than 1,300 reports of civilian casualties obtained by The Times, showed that the air campaign against the Islamic State was marked by flawed intelligence, confirmation bias and scant accountability.

“The United States should not contribute in any way to the suffering of millions of innocent Yemenis caught in a devastating Saudi-led war,” Ms. Warren said. “The U.S. government has a moral and legal obligation to ensure its actions are not exacerbating a dire humanitarian crisis, and there is strong bipartisan support for thorough investigations into possible U.S. complicity to civilian harm in Yemen.”