Saudi Arabia has warned criticism of its Crown Prince Mohammed bin Salman is a "red line", after US President Donald Trump praised the kingdom in defiance of concerns that he was giving Riyadh a pass on a writer's murder.

Saudi Foreign Minister Adel al-Jubeir said on Wednesday that calls for the crown prince to be held accountable for the killing of Jamal Khashoggi would not be tolerated.

His comments came as the US president praised Saudi Arabia for keeping oil prices low - one strand of his argument against punishing Riyadh even though the CIA reportedly found strong evidence that the crown prince was involved in the murder.

"In Saudi Arabia our leadership is a red line. The custodian of the two holy mosques (King Salman) and the crown prince are a red line," Jubeir told the BBC.

"They represent every Saudi citizen and every Saudi citizen represents them. And we will not tolerate any discussion of anything that is disparaging toward our monarch or our crown prince."

Khashoggi, a US resident who wrote for The Washington Post and had been critical of Riyadh, was lured to the Saudi consulate in Istanbul on Oct 2, killed and reportedly dismembered.

After lengthy denials, Saudi authorities admitted responsibility and said 21 people had been taken into custody. However, a CIA analysis leaked to the US media went further, reportedly pointing the finger at the crown prince.

But Trump, on holiday at his Florida Mar-a-Lago Club on Wednesday, doubled down on a statement from Tuesday that he was essentially ignoring the killing of Khashoggi because of what he said were more important US strategic and commercial interests.

"Oil prices getting lower. Great! Like a big Tax Cut for America and the World. Enjoy! $54, was just $82," he tweeted. "Thank you to Saudi Arabia, but let's go lower!"

Jubeir insisted that the crown prince had not been involved in the killing.

"We have made that very clear. We have investigations ongoing and we will punish the individuals who are responsible for this," he said.

He called on Turkey to come forward with all its evidence about the slaying and stop leaking information.

The foreign minister said the murder was a "rogue operation" by intelligence officers.

Jubeir also said any possible US sanctions on Saudi Arabia would be short-sighted.

Meanwhile, Turkish President Recep Tayyip Erdogan may meet with Saudi crown prince during a visit to Argentina for the G20 summit, his spokesman said on Thursday.

AFP and Reuters contributed to this story.