(Adds Mattis' comments in paragraphs 11-12, 14)
WASHINGTON, Nov 21 (Reuters) - U.S. President Donald Trump on Wednesday praised Saudi Arabia for helping to lower oil prices as pressure intensified for the United States to impose tougher sanctions on its Middle East ally over dissident Saudi journalist Jamal Khashoggi's murder.
In a tweet, Trump thanked Riyadh for the recent drop in oil prices and called for prices to go even lower to boost the U.S. and global economies.
Trump has repeatedly blasted high oil prices, criticized OPEC over its production and pressured Saudi Arabia, a major oil producer, to act.
Oil prices rose on Wednesday but have been trending lower for weeks. Oil prices tumbled more than 6 percent on Tuesday, with U.S. crude diving to its lowest in more than a year, caught up in a Wall Street selloff fed by mounting concerns about slowing global growth.
"Oil prices getting lower. Great! Like a big Tax Cut for America and the World. Enjoy! $54, was just $82. Thank you to Saudi Arabia, but let's go lower!" Trump wrote.
The Organization of the Petroleum Exporting Countries and its allies are due to meet in two weeks and are expected to curb production to stem rising supplies.
OPEC and its allies, led by Russia, are discussing a proposal to cut output by 1.4 million barrels per day (bpd), sources told Reuters last week.
Lower output would reverse months of increased production which saw Saudi and Russian oil production hit record highs.
Brent crude fell from $86.74 a barrel in October to $61.71 on Tuesday, its lowest since December 2017. U.S. WTI crude futures slumped nearly 8 percent on Tuesday to $52.77 having hit a four-year high of $76.90 last month.
On Tuesday, Trump pledged to stand by Saudi Arabia even as he said that Saudi Crown Prince Mohammed bin Salman may have known about the plan to murder Khashoggi last month.
U.S. Defense Secretary Mattis on Wednesday said the United States had a twin principle of accountability and the pursuit of human rights while also continuing to work with Saudi Arabia to end the war in Yemen.
"On the Khashoggi affair, presidents don't often get the freedom to work with unblemished partners in all things," Mattis told reporters.
The CIA believes Khashoggi's death was ordered directly by the crown prince, Saudi Arabia's de facto ruler, sources familiar with the matter have said.
Mattis said he did not think the CIA or the Saudi government had fully established who was responsible for the killing.
With U.S. lawmakers calling for tougher sanctions, Trump said he would not cancel military deals with the kingdom. He said it would be a "foolish" move that would only benefit Russia and China, competitors of the United States in the arms market.
A number of U.S. lawmakers, including some of Trump's fellow Republicans, pushed back sharply on his assessment and urged further congressional action.
U.S. Secretary of State Mike Pompeo again defended Trump's stance on Wednesday, saying the United States had already sanctioned 17 Saudis and that Riyadh was critical to the administration's Iran strategy.
"Not every country shares our value set," he told local radio station KQAM. "Our mission statement is to make sure that the American people are safe and secure." (Reporting by Susan Heavey and Idrees Ali in Washington and Ahmad Ghaddar in London; Editing by Tom Brown and James Dalgleish)
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