WASHINGTON — Democratic lawmakers warned Tuesday they will work to block munition sales to Gulf allies in response to civilian casualties in the Saudi-led intervention in Yemen, even as Defense Secretary Jim Mattis defended U.S. military support to the coalition.

Mattis’ remarks to reporters at the Pentagon on Tuesday came as a new United Nations report said all sides in the conflict may have committed war crimes. Following a strike on a school bus that killed 40 children earlier this month, opposition on Capitol Hill to military aid has deepened, with frustration the Pentagon cannot better explain its support.

“The Department of Defense can’t give you a good answer about how it is our assistance is actually making it better in terms of civilian casualties,” said Senate Foreign Relations Committee ranking member Sen. Robert Menendez, D-N.J. “It’s hard to explain that hitting a school bus is something we’re doing that is a better result."

Menendez said he is continuing to block the Trump administration from a round of sales of precision-guided munitions to Gulf nations. That’s because he has been dissatisfied with the Pentagon’s response to his inquires, particularly when it comes to the U.S. military’s actions after a coalition strike, he said.

Sen. Chris Murphy, D-Conn., and a foe of U.S. participation in the conflict, said Tuesday he is ready to introduce a joint resolution of disapproval in the Senate to reject the sale should the Trump administration advance it to the formal notification stage. (The administration has informally notified Congress, which allows lawmakers to delay the sale over their questions and concerns for a finite period.)

In March, Murphy offered a similar resolution of disapproval with Sen. Mike Lee, R-Utah, and Bernie Sanders, I-Vt., to end U.S. involvement. It failed in a bipartisan vote, 55-44. On Tuesday, Murphy and other Democrats said congressional opposition is now stronger, particularly since the bus bombing and the U.N. findings.

“I don’t know why people don’t believe their eyes,” Murphy said. “More civilian casualties, less chance of peace, humanitarian disaster getting worse. We need to cut our losses.”

For three years, the U.S. has provided munitions, aerial refueling and other logistical support to Saudi and Emirati forces fighting in the Yemeni civil war against Houthi rebels alleged to be Iranian-backed. At the Pentagon, Mattis told reporters U.S. support is “not conditional” and urged the coalition to “do everything humanly possible to avoid any innocent loss of life.”

A team of U.N.-mandated investigators said Tuesday that air attacks by the Saudi and Emirati-led coalition has caused the most direct civilian casualties in the war, and a blockade of Yemeni ports and airspace may have violated international humanitarian law. Those attacks have reportedly hit residential areas, markets, funerals, weddings, detention facilities, civilian boats and medical facilities.

The school bus bombing has also put a spotlight on U.S. weapons sales to the region. CNN reported earlier this month that the bomb that hit the bus was a 500-pound laser-guided MK 82 bomb made by Lockheed Martin. In 2016, U.S.-supplied bombs separately struck a funeral hall in Yemen, killing 155 people, and a Yemeni market, killing 97.

At the Pentagon, Mattis said U.S. support is meant to bring about a U.N.-brokered peace. U.S. training has yielded successes in averting civilian casualties, he said, like the establishment of “no-fire zones” around schools and hospitals, and coalition “pilots in the air who have declined to drop [bombs] even when they have the authority.”

After the strike on the school bus, Mattis ordered a three-star general to Riyadh to voice concerns about coalition air operations to Saudi officials. Mattis said Tuesday that U.S. partners have always been receptive: “We have not seen any callous disregard by the people we’re working with.”

The coalition has improved targeting before a sortie is launched, but it needs to improve “dynamic” targeting, meaning while an aircraft is in flight — as was the case in the school bus bombing, according to Mattis.

Soon after Trump took office in 2017, his administration reversed a decision by former President Barack Obama, driven by civilian deaths in the war, to suspend the sales of laser-guided bombs to Riyadh. The Senate narrowly approved that sale 53-47.

The Senate Armed Services Committee’s top Democrat, Sen. Jack Reed, said Tuesday that part of the Defense Department’s rationale for refueling coalition aircraft is that it provides more time for targeteers to make calculated decisions. But he said the Pentagon has not been entirely forthcoming with lawmakers.

“I don’t think we’ve gotten yet a very detailed analysis of what took place, of what they knew and what they should have known,” Reed said of coalition forces. “Do they have active means to measure accountability — not to us because we’re not involved in the operations directly, but we don’t want to be in a position where [we’re] involved in something that’s not being run according to the rule of law.”

SASC member Sen. Elizabeth Warren, D-Mass., sent a letter to U.S. Central Command’s commander Gen. Joseph Votel earlier this month, asking whether the U.S. military can track the purpose, mission and results of airstrikes in Yemen it supports. Thursday is her deadline for him to answer.

It’s unclear whether a wide array of Republicans, who tend to favor the U.S. support, share those concerns. On Tuesday, Senate Foreign Relations Committee member Sen. Marco Rubio, R-Fla., said U.S. efforts mean to stem Iran’s expanding influence in the region and provide “better targeting to avoid hitting civilians and innocents.”

Munitions sales support that end, he said.

“In the absence of U.S. assistance and that technology, all you’re going to see is untargeted bombings and kinetic military activity that potentially harms civilians,” Rubio said. “You hate to see innocent people killed in a war zone, but I blame the Houthis for that because they’re the ones who have created this situation.”

Senate Foreign Relations Committee Chairman Bob Corker, R-Tenn., has previously voiced his doubts about the strategy behind U.S. aid. In a brief hallway interview, he was not as quick to defend U.S. support for the coalition, saying only: “We have friends involved, but I’m not sure anybody’s conduct is particularly good at this time.”