US warns support for Saudi-led coalition in Yemen war not 'unconditional'

US support for the Saudi-led coalition fighting in Yemen is not "unconditional", the country's defence secretary said on Tuesday, as he defended Washington's role in the ongoing war.

Jim Mattis' 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 Saudi-led coalition's investigations into alleged war crimes in Yemen have lacked credibility and failed to provide redress to civilian victims, added the 90-page report.

Washington provides weapons, aerial refueling to jets, intelligence and targeting information to the Saudi-led coalition that is fighting Houthi 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."

But, he said, US support has 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".

The devastating Yemen conflict has left at least 10,000 people dead since March 2015, when the Saudi-led coalition intervened to fight Houthi rebels closing in on the last southern bastion of President Abd Rabbo Mansour Hadi's government.

The UN human rights office said Tuesday that some 6,660 civilians were among the dead, while more than 10,500 had been injured.

Mattis described the complexity of the war, noting that rebels have launched weapons from residential areas into Saudi Arabia.

"We have been working with the Saudis and the Emirates doing what we can to reduce any chance of innocent people being injured or killed," he said. 

"At no time have we felt rebuffed or ignored when we bring concerns to them," he went on, noting that he had not seen any "callous disregard" from the coalition. 

The Pentagon chief added that training given to coalition pilots is paying off.

"We have had pilots in the air who recognise the danger of a specific mission and decline to drop, even when they get the authority," he said. 

"We have seen staff procedures that put no-fire areas around areas where there's hospitals or schools."