US support to coalition in Yemen not unconditional: Mattis

US support for the Saudi-led coalition fighting in Yemen is not "unconditional", Defence Secretary Jim Mattis today said, as he defended America's ongoing role in the war.

The Pentagon chief's comments came the same day as UN investigators said they had reasonable grounds to believe that warring parties in Yemen may have committed a "substantial number" of violations of humanitarian law that could amount to "war crimes".

The US provides weapons, aerial refuelling to jets, intelligence and targeting information to the Saudi-led coalition that is fighting Huthi rebels in Yemen.

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

Those deaths came after a coalition attack on a bus in the northern rebel stronghold of Saada early this month 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 Pentagon reporters.

"That is our goal where we engage with the coalition."
"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."