Defense Secretary Jim Mattis said on Tuesday that the US intends to continue backing the Saudi-led coalition fighting Iranian-backed Houthi rebels in Yemen.

The US marginally supports the Saudi-led coalition, despite several reports and accusations of war crimes and human rights abuses.

Criticism of the Saudi campaign surged earlier this month when an airstrike hit a bus, killing dozens, including school children.

Defense Secretary Jim Mattis said on Tuesday that the US intends to continue backing the Saudi-led coalition fighting Iranian-backed Houthi rebels in Yemen despite civilian casualties and questions about the Saudis' commitment to avoiding killing innocents.

At a rare Pentagon news conference, Mattis defended US support for the coalition, saying American influence on the Arab air campaign has made a difference in reducing instances of errant bombing and the targeting of civilians.

He noted, however, that US support is conditioned on a Saudi commitment to doing "everything humanly possible" to avoid any loss of innocent life and Riyadh supporting a UN-brokered peace process to end the civil war. The US provides the Saudis and their United Arab Emirate coalition partners with intelligence, aerial refueling and military advice, but US forces are not directly involved in the airstrikes or other aspects of the fighting.

"For the last several years we have been working with the Saudis and the Emiratis, doing what we can to reduce any chance of innocent people being injured or killed," Mattis said.

Mattis said the US supports Saudi Arabia's right to defend its territory against rocket attacks by Houthi rebels in Yemen. He said the Saudi military has been receptive to US advice and training on conducting airstrikes.

"At no time have we felt rebuffed or ignored when we bring concerns to them," he said. "The training that we have given them we know has paid off." He cited instances of Saudi pilots deciding during a combat mission to withhold fire to avoid potential civilian casualties, even when they have authority to fire.

Criticism of the Saudi campaign surged earlier this month when an airstrike hit a bus, killing dozens, including school children. The bomb used in the airstrike on August 9 was reportedly a 500-pound (227 kilogram) laser-guided MK 82 bomb made by Lockheed Martin, a top US defense contractor.

Last Thursday, the Saudi-led coalition allegedly killed another 30 people, including 22 children, in multiple airstrikes.

Mattis noted the US is pressing the Saudi government to complete an investigation of what went wrong.

"We recognize every mistake like this is tragic in every way, but we have not seen any callous disregard by the people we're working with," Mattis said. "So we will continue to work with them."

Among those in Congress calling for the US to pull the plug on support for the coalition is Rep. Ro Khanna, a California Democrat, who has called the bus attack a barbaric act. On the day of the attack he said the US "must end our complicity in this slaughter."

In January, Germany and Norway announced that they would stop selling weapons to countries in the Saudi-led coalition over the war in Yemen.

Three experts working for the UN's Human Rights Council said in a report this week that the governments of Yemen, the United Arab Emirates and Saudi Arabia may have been responsible for war crimes during 3½ years of fighting against rebels there, including rape, torture, arbitrary detention, using child soldiers, and more.

Such reports and allegations of war crimes and human rights abuses are certainly not new.

The Saudi-led coalition has also repeatedly been accused of conducting indiscriminate and unlawful airstrikes, as well as blocking food, fuel, and medicine into Yemen, according to Human Rights Watch. At the same time, the Houthis have repeatedly been accused of firing artillery at Yemeni cities, missiles at Saudi Arabia, using child soldiers, and more.

Last week, the international rights group Human Rights Watch charged that the Saudi-led coalition had not credibly investigated civilian casualties. It said the coalition's "sham investigations" have fallen short of "international standards regarding transparency, impartiality, and independence." The report highlighted discrepancies between the findings of the coalitions' investigative body, the Joint Incidents Assessments Team, and those by Human Rights Watch.