Defending America’s ongoing role in the war in Yemen, US Defense Secretary Jim Mattis said on Tuesday that his country’s backing for the Saudi-led coalition operating there was not “unconditional.”

The Pentagon chief’s comments came on the same day as United Nations  investigators said they believed warring parties in Yemen might have committed a “substantial number” of violations of humanitarian law that could be considered “war crimes.”

The US provides weapons, aerial refueling to jets, intelligence and targeting information to the Saudi-led coalition fighting Yemen’s Houthi rebels.

Last week, the UN said two air raids killed at least 26 children and four women south of the insurgent-held city of Hodeida.

That tragedy followed a coalition attack on a bus in the northern insurgent stronghold of Saada early this month that killed 40 children.

“Our conduct there is to try and keep the human cost of innocents being killed accidentally to the absolute minimum,” Mattis told journalists. “That is our goal where we engage with the coalition.”

However, he said US support had its limits.

“It is not unconditional,” he said, noting that the coalition must do “everything humanly possible to avoid any innocent loss of life, and they support the UN-brokered peace process.”