Yemenis attend a funeral Monday after dozens of victims, including many children, died in a Saudi-led airstrike in Saada last week. (Hani Mohammed/AP)

Lawmakers increased pressure on the Trump administration over the war in Yemen this week after an airstrike killed dozens of children, urging officials to explain and possibly adjust U.S. support for nations waging war against rebels there.

Democratic members of the House and Senate sent three letters in as many days to administration officials, asking for an accounting of U.S. involvement in a conflict critics say has exposed Washington to claims of responsibility for thousands of civilian deaths.

A Democratic senator, meanwhile, announced a measure on Friday that would halt military assistance to Saudi Arabia, the United Arab Emirates and other nations battling Yemen’s Houthi rebels until the administration can determine their air campaign has not violated international law.

While the United States has provided intelligence support and aerial refueling to the Saudi-led coalition since it began its intervention in 2015, officials have sought to distance themselves from Gulf pilots’ repeated strikes on Yemeni civilians and the mounting humanitarian crisis the war has spawned.

In a letter on Monday to Gen. Joseph Votel, head of U.S. Central Command, Sen. Elizabeth Warren (D-Mass.) made reference to the bombing of a bus in Yemen’s northern Saada province on Aug. 9, which local officials said killed at least 40 children.

In her letter, Warren asked Votel to clarify his recent assertion that Centcom is unable to determine whether the United States assisted specific coalition flights that resulted in civilian deaths. A report in the Intercept alleged that U.S. intelligence officials had detailed information about one such strike.

In another letter Tuesday, 30 House Democrats asked Defense Secretary Jim Mattis, Secretary of State Mike Pompeo and Daniel Coats, director of national intelligence, for a briefing about U.S. involvement in the war, including efforts to avert a battle in the coastal city of Hodeidah, a key humanitarian portal.

“The ongoing military escalation threatens millions of civilians, and hundreds of thousands are at risk of starvation,” Minority Whip Steny H. Hoyer (D-Md.) said in a statement to The Washington Post. “The United States must have a cohesive strategy to address the situation.”

Rep. Adam Smith (Wash.), the top Democrat on the House Armed Services Committee, said that at a time of crisis in Yemen “close congressional oversight is absolutely critical.”

In a third letter, Rep. Ted Lieu (D-Calif.), a former Air Force lawyer, on Monday appealed to the Defense Department inspector general to open an investigation into whether American military personnel have violated U.S. or international law by supporting the Saudi-led coalition.

“I am deeply concerned that continued U.S. refueling, operational support functions and weapons transfers could qualify as aiding and abetting these potential war crimes,” he wrote.

The letters are the most recent expression of mounting congressional frustration about the war, most prominently among Democrats but also Republicans.

On Monday, President Trump signed a defense authorization law containing a bipartisan measure that requires the administration to ensure that Gulf nations are taking steps to protect civilians and end the war before the Pentagon can continue its support.

“Our humanitarian principles and our national security interests require that the United States use its influence to end the civil war in Yemen and address the world’s largest humanitarian crisis,” Sen. Todd C. Young, an Indiana Republican who sponsored that measure, said in a statement.

Separately, Sen. Chris Murphy (D-Conn.) unveiled a proposed amendment to a defense appropriations bill that would link U.S. aid to ensuring the coalition has not run afoul of international law or U.S. policy on protecting civilians. It is not clear whether the amendment will be adopted.

“Either the Pentagon should be 100 percent certain that U.S. weapons and funding aren’t being used to commit war crimes in Yemen, or we should cut off U.S. support right now,” Murphy said in a statement.

Congress has also focused on weapons sales in an effort to limit U.S. involvement in the war. New Jersey Sen. Robert Menendez, the top Democrat on the Senate Foreign Relations Committee, has placed a hold on a proposed sale of precision munitions to Saudi Arabia and the UAE.

The bus strike killed children on an end-of-summer excursion and galvanized opposition to the coalition’s conduct within the Trump administration, which has voiced strong support for its Gulf allies and echoed concerns about Iran’s links to the Houthis.

According to U.S. officials, Saudi military personnel, who have publicly described the strike as a “legitimate military action,” told U.S. diplomats they targeted the bus because they believed it had two Houthi leaders aboard.

A spokesperson for the National Security Council, who provided a statement on the condition of anonymity, said the administration would look forward to “timely and transparent” results from an ongoing Saudi investigation of the strike.

“All of our assistance to the Saudi-led coalition is predicated on their continued adherence to the Law of Armed Conflict and International Humanitarian Law,” the spokesperson said. “We have been working with Saudi Arabia and the UAE to help them further minimize civilian casualties.”

A senior U.S. official, speaking to reporters in Cairo on Wednesday on the condition of anonymity, said results of the Saudi-led investigation were expected in days but expressed doubts about the coalition’s ability and willingness to complete a probe in the desired manner.

When asked whether the U.S. military could have refueled the warplane that launched the strike in Saada, the official said it was possible but not certain. When asked why the United States could not determine this or whether a U.S. bomb was used, the official said “we would have to have Saudis provide us information, but they don’t in the normal course of events.”

When pressed on why the United States could not ask Saudi Arabia, a close ally and major purchaser of U.S. arms, for such information, the official said it would require more U.S. manpower to oversee the Saudi inventory and how they use it. The official questioned whether anything would be gained by knowing the details of the U.S. role in each specific airstrike.

“There are people asking the question,” the official said. “Well, what difference does that make? We are providing the refueling and support to Saudi aircraft. We are also selling them munitions that are being used in these We are not denying that.”

Sudarsan Raghavan in Cairo contributed to this report.