US President Donald Trump has said Saudi authorities have staged the "worst ever cover up" in the death of journalist Jamal Khashoggi.

Saudi Arabia has admitted that Khashoggi, 59, a contributor for The Washington Post, was killed in its Istanbul consulate.

"They (the Saudis) had a very bad original concept. It was carried out poorly. And the cover-up was one of the worst in the history of cover-ups.

"Very simple. Bad deal. Should have never been thought of. Somebody really messed up. And they had the worst cover-up ever," Trump told reporters in the Oval Office Tuesday.

Khashoggi, a former royal family insider turned critic of the Saudi crown prince, disappeared after he entered the consulate on October 2 to collect a document for his upcoming marriage.

The incident has severely dented the international reputation of the Crown Prince, Mohammed bin Salman.

Responding to a question on the probe into the death of Khashoggi, the US president said, “And where it should have stopped is at the deal standpoint, what they thought about it. Because whoever thought of that idea, I think, is in big trouble. And they should be in big trouble."

Turkish President Recep Tayyip Erdogan has said the "savage murder" of the journalist was meticulously planned, and demanded that all those linked to the killing face punishment.

He, however, acknowledged Turkey was still searching for answers to key questions, including who gave the order.

Trump told reporters that Erdogan was pretty tough on Saudi Arabia.

“He was pretty rough on Saudi Arabia, I would say. I haven't gotten a full recap, as you know. I have people in Turkey and I have people in Saudi Arabia and other places, and they're all coming back as we speak, they're heading back,” he said.

CIA Director Gina Haspel is currently in Turkey and is expected to be back soon in the US.

“I'll know, I think, everything in a very short period of time. It's a bad situation. But certainly, president Erdogan was not complimentary of what happened. That was a terrible thing that happened,” he said.

Trump reiterating that he does not want to scrap the USD 110 billion mega arms deal with Saudi Arabia and the total investment of USD450 billion from that country, but did not rule out not taking any action against the Saudis.

“I want to see the facts first. Look, Saudi Arabia's been a really great ally. They've been one of the biggest investors, maybe the biggest investor, in our country.

"They are doing hundreds of billions of dollar’s worth of investments. And, you know, so many jobs. So many jobs. Thousands and thousands of jobs,” he said.

During his visit to Saudi Arabia in May 2017, Trump signed an agreement with the Saudis for them to purchase USD 110 billion of US weapons, although so far only USD 43 billion of that has been detailed.

“The ultimate number is around USD450 billion. I think that's over a million jobs. A million to over a million jobs. So we do that, we're just hurting ourselves. We're just hurting ourselves,” Trump said.