The Saudi-led coalition fighting in Yemen said early Saturday it had "requested cessation of inflight refuelling " by the US for its fighter jets after American officials said they would stop the operations amid growing anger over civilian casualties from the kingdom's airstrikes.
Mr Mattis argued that halting USA military support could increase civilian casualties, since U.S. refuelling had given pilots more time to select their targets.
According to the United Nations, some 14 million Yemeni people - fully half the country's population - are dependent on food aid for their survival, and more than 400,000 children are suffering from serious malnutrition.
The Saudi-led military operations are "increasingly confining populations and cutting off exit routes", the statement said.
David Miliband, who is a former British foreign secretary and member of parliament, said while the journalist's death was tragic, global focus on Khashoggi's murder should be switched to actions of Saudi Arabia in Yemen, where millions of lives are affected.
Members of Congress have been calling for the administration to curtail arms sales to Saudi Arabia or take other action.
The refueling decision, which was first reported by The Washington Post, has been under discussion for a few weeks.
"We have one clear demand and that is a complete halt to Saudi airstrikes", said one Houthi official who spoke to The Associated Press on Saturday on condition of anonymity because he was not authorized to brief the media.
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Mattis acknowledged "continued bipartisan interest from Congress", and said the Trump administration is "appreciative of the continued dialogue we have had with key members on this issue".
U.S. officials told Reuters only a fifth of Saudi-led coalition aircraft require in-air refuelling from the United States.
However, a halt to refuelling could have little practical effect on the conflict, seen as a proxy war between Saudi Arabia and Iran. USA officials told Reuters only a fifth of Saudi-led coalition aircraft require in-air refueling from the United States.
"The U.S. and the Coalition are planning to collaborate on building up legitimate Yemeni forces to defend the Yemeni people, secure their country's borders, and contribute to counter Al Qaeda and ISIS efforts in Yemen and the region", he said in a statement. The US and United Kingdom have both been criticized for continuing to sell arms to the coalition despite their targeting of civilians and alleged war crimes. It said that the coalition, which relies heavily on air power, has killed scores of civilians in recent airstrikes, and rebels are responding with mortars in residential neighborhoods that cause indiscriminate casualties.
In August the defense secretary warned that USA support for the coalition was "not unconditional", noting it must do "everything humanly possible to avoid any innocent loss of life".
US Representative Ro Khanna, an advocate of barring US military support to the Saudi war on Yemen, has reportedly introduced a measure in the House to ensure the Trump administration follows through on its decision.
In the last month, the UAE has mounted an all-out offensive to capture the critical Yemeni port of Hodeida. "It could avert a humanitarian crisis", he told U.S. online news publication The Intercept.
According to experts, more than 22 million people in are in desperate need in what is already the Arab world's poorest country.
Human rights groups say the real death toll could be five times as high.