Militiamen in a front-line position on the northern side of Taiz, Yemen. The front line that encircles the city has not moved significantly in the past two years. There are sporadic exchanges of fire, but neither of the warring parties is seriously pushing to gain territory. (Lorenzo Tugnoli/For The Washington Post)

Lawmakers pressed the Trump administration Tuesday over its continued involvement in Yemen’s civil war, as widespread civilian suffering and staunch White House support for Saudi Arabia’s campaign there fuel opposition on Capitol Hill.

Senators’ skeptical questioning of high-level nominees provided new indications of how the administration’s defense of Saudi Arabia, despite the punishing Yemen conflict and the kingdom’s role in the death of journalist Jamal Khashoggi, has become a defining issue for bipartisan critics of President Trump’s foreign policy.

Since 2015, the United States has provided military support, including intelligence sharing, logistics help and, until last month, refueling of combat aircraft, to Saudi Arabia and other nations battling Iranian-linked Houthi rebels in Yemen.

The war has triggered the world’s worst humanitarian crisis, leaving millions of Yemenis unable to access adequate food and medical care. Critics warn that continued U.S. aid to the Saudi-led coalition will perpetuate the war and possibly allow millions more to slip into deprivation.

Marine Lt. Gen. Kenneth McKenzie Jr., Trump’s nominee to lead U.S. Central Command, argued the opposite view, telling members of the Senate Armed Services Committee that U.S. involvement advances the goal of a peace deal between the Houthis and the internationally backed Yemeni government.

“I believe our ability to participate and drive those discussions requires that we remain in contact with both [the United Arab Emirates] and the kingdom of Saudi Arabia,” he said. The UAE is also conducting operations against Houthi positions in Yemen.

McKenzie spoke less than a week after the Senate voted to advance legislation that would force the military to end its role in the Houthi conflict. While opposition has been growing quietly for months, critics have been galvanized by the Trump administration’s backing of Saudi leaders after the gruesome murder and dismemberment of Khashoggi, a Washington Post contributing columnist who was a prominent critic of the royal family.

That legislation is expected to be taken up again next week.

Sen. Tim Kaine (D-Va.) voiced exasperation at what he called a “credibility issue” rooted in the administration’s depiction of its role in the war. “We have had military leaders look at the Senate and say that we are not involved in hostilities to support either side in the Yemeni civil war,” Kaine said.

“One of the reasons that we’re having this debate . . . is we don’t like being told — and we’re proxies for the American public — we don’t like being told we’re not involved in hostilities, when bombs that are falling are made in the United States and when U.S. jets are involved in refueling Saudi bombing runs into Yemen,” he said. “We’re insulted by that. . . . We don’t find that to be believable.”

Pentagon lawyers have long asserted that the U.S. role in Yemen does not meet the legal threshold for involvement in hostilities.

Lawmakers had a second opportunity to air their views hours later when Christopher Henzel, nominated to be the next U.S. ambassador to Yemen, appeared before the Senate Foreign Relations Committee. Henzel now serves as the charge d’affaires at the U.S. Embassy in Riyadh, Saudi Arabia.

Questions about Yemen dominated a hearing to confirm Henzel and a spate of other diplomatic nominees.

The testimonies occurred as Yemen’s warring parties prepared to attend U.N.-brokered peace discussions in Sweden, which, if successful, would be a positive milestone in an otherwise bleak outlook.

Henzel, like McKenzie, defended U.S. support for the coalition, which he said had taken steps to reduce civilian casualties caused by air operations targeting the Houthis. Critics argue that little meaningful progress has been made.

American involvement, the nominees said, remains important in setting conditions conducive to a parallel campaign against Islamist extremists who have sought to target the West, an operation that has been a priority for the Pentagon.

McKenzie said al-Qaeda in the Arabian Peninsula (AQAP) had been “significantly degraded” in counterterrorism operations over the past year.

While most of the criticism came from Democrats, several Republican lawmakers joined in questioning the administration’s approach in the Gulf, including Sen. Todd Young (R-Ind.), who told Henzel he believed the United States had not effectively used its leverage over Saudi Arabia to secure an end to the fighting.

The testimony provided further indications that the Trump administration shares Persian Gulf nations’ view that Yemen has become a key theater for Iran’s attempt to expand its influence in the Middle East.

“The war in Yemen has complicated our ongoing counterterrorism efforts and has facilitated Iran’s ambitions, further threatening regional stability,” Henzel said in his prepared testimony.

Last week U.S. officials displayed new weaponry that they said shows Iran’s support for the Houthis, who have fired missiles into Saudi Arabia and attacked ships at sea.

The hearings took place as CIA Director Gina Haspel made a separate appearance to brief key senators on U.S. intelligence related to Khashoggi’s death.

While the kingdom has identified a group of men it says were responsible for killing Khashoggi, a growing group of lawmakers contends that the administration needs to do more to determine what hand Crown Prince Mohammed bin Salman may have had. Saudi authorities have said it was a rogue operation.

McKenzie, who testified alongside Army Lt. Gen. Richard Clarke, the nominee to lead U.S. Special Operations Command, also characterized the war in Afghanistan as “largely stalemated” despite 17 years of U.S. and allied involvement. He voiced confidence that conditions were aligning in favor of peace talks with the Taliban but warned that Afghan forces were probably not able to defend the country successfully if foreign forces departed precipitously.

Clarke, for his part, said he would work to adapt America’s Special Operations force to meet the Pentagon’s new mandate to compete more effectively with Russia and China, while maintaining its nearly 20-year focus on extremist groups.

Karoun Demirjian and Shane Harris contributed to this report.