Prime Minister Theresa May has announced that the United Kingdom will ban all those suspected of involvement in the death of Jamal Khashoggi from the country.

Mr Khashoggi died after entering the Saudi embassy in Istanbul, with the country originally claiming he was killed in a fist-fight, something Mrs May says is not a credible explanation.

Mrs May admitted the UK would not go as far to stop selling arms to the Saudis.

"We condemn the killing of the journalist Jamal Khashoggi in the strongest possible terms and after his disappearance we made clear that Saudi Arabia must co-operate with Turkey and conduct a full and credible investigation," Mrs May said.

"The claim that has been made that Mr Khashoggi died in a fight does not amount to a credible explanation so there does remain an urgent need to establish what has happened in relation to this."

She added that she would speak to King Salman on Wednesday and Home Secretary Sajid Javid is "taking action against all suspects to prevent them entering the UK".

"If these individuals currently have visas, those visas will be revoked today," she told MPs.

US president Donald Trump has described the killing of Mr Khashoggi as one of "worst cover-ups in the history of cover-ups".

In light of the death in Istanbul of the high profile critic of the Saudi regime, the US will revoke the visas of some Saudi officials who have been implicated in Khashoggi's killing.

But Mr Trump struck a cautious note over punishing Saudi Arabia, one of America's biggest investors, saying he was still gathering information from American officials in Turkey and Saudi Arabia.

"I want to see the facts first. Look, Saudi Arabia has 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 investments, and so many jobs, thousands and thousands of jobs," he told reporters.

He went onto say that "whoever thought of that idea (of killing Khashoggi) is in big trouble".

"It was carried out poorly and the cover-up was one of the worst cover-ups in history. It's very simple," Mr Trump said.

Mr Trump told the Wall Street Journal he thought it was possible that Saudi crown prince Mohammed bin Salman could be involved with Khashoggi's death.

“Well, the prince is running things over there more so at this stage. He’s running things and so if anybody were going to be, it would be him.”

Prince Mohammed was questioned by Mr Trump regarding the journalist's death but stated he had no involvement in the matter.

“My first question to him was, ‘Did you know anything about it in terms of the initial planning’,” Trump said. Prince Mohammed replied that he didn’t, Trump said.

“I said, ‘Where did it start?’ And he said it started at lower levels.”

After criticising Iran, Mr Trump took a swipe at the Middle East, describing the region as a "nasty place, it's a nasty part of the world".

Earlier on Tuesday, Turkey's president Recep Tayyip Erdogan had accused Saudi officials of planning the journalist's days before his death as other world leaders condemned the kingdom's regime for the killing.

Saudi Arabia claimed on Saturday, Mr Khashoggi was killed in the consulate in a "fistfight" after initially insisting he had left the building alive after a visit to collect papers for his forthcoming marriage.

Erdogan warned diplomatic immunity was not "armour" for murder, and he called on the Saudi king to allow 18 suspects to be tried in Turkish courts.

The regime must reveal who planned the killing, regardless of their rank, he added.

Hours after Erdogan's statement on Tuesday, King Salman and heir-apparent Crown Prince Mohammed bin Salman met members of the journalist's family to offer their condolences.

The Saudi Foreign Ministry posted pictures of the royal family shaking hands with Sahl bin Ahmed Khashoggi and his son, Salah bin Jamal Khashoggi at the Yamama Palace in Riyadh watched by armed guards.

Salah has been banned from leaving Saudi Arabia since last year because of his father's criticisms of the kingdom's government.

Erdogan accused Saudi officials of planning the journalist's days before his death Credit: AP

A stream of leaks to national and international media has increased pressure on Saudi Arabia, which is hosting an investment conference this week that many dignitaries have decided to skip because of the scandal.

Foreign secretary Jeremy Hunt joined other G7 foreign ministers in condemning the killing Khashoggi, "in the strongest possible terms".

In a joint statement, the ministers said the confirmation of Mr Khashoggi's death by the Saudi authorities was "a first step toward full transparency and accountability" but that their explanations left "many questions unanswered".

"Those responsible for the killing must be held to account. Saudi Arabia must put in place measures to ensure something like this can never happen again."

Speaking on Tuesday at a Washington Post event, US Vice President Mike Pence said Mr Khashoggi's death "will not go without an American response." His statement comes after president Donald Trump said he was not satisfied with the explanations he has heard about Mr Khashoggi's death despite previously telling a reporter he thought they sounded credible.

Jamal Khashoggi was a critic of the Saudi regime and lived in the US. Credit: AP

Saudi Arabia said 18 Saudis had been arrested and several top intelligence officials had been fired over the killing, but critics claim the punishment was designed to absolve Crown Prince Mohammed bin Salman of any responsibility.

A gruesome account in Turkey’s Yeni Safak newspaper has alleged Saudi officials cut off Mr Khashoggi’s fingers and then decapitated him inside the consulate while his fiancee waited outside.

Leaked CCTV footage appeared to show a "body double" wearing Mr Khashoggi's clothes just hours after he was killed.

The video, broadcast by CNN, shows the man strolling out of the Saudi consulate in Istanbul where the journalist was killed.

Jamal Khashoggi entering the consulate.

Secretary of State Mike Pompeo said a move to revoke visas was just a first step.

Visa records are confidential and Mr Pompeo was not more specific about who the revocations would affect, but the State Department later said 21 "Saudi suspects" would have visas revoked or would be declared ineligible to enter the US.

"These penalties will not be the last word on this matter," Mr Pompeo told reporters at the State Department.

The administration "will continue to hold those responsible accountable. We're making very clear that the United States does not tolerate this kind of ruthless action to silence Mr Khashoggi, a journalist, with violence," he said. "Neither the president or I am happy with this situation."

Still, Mr Pompeo stressed the strategic importance of the US-Saudi relationship.

"We continue to view as achievable the twin imperative of protecting America and holding accountable those responsible for the killing of Mr. Khashoggi," Mr Pompeo said.