Turkey's president kept up pressure on Saudi Arabia on Wednesday as the kingdom's powerful crown prince was to address an international investment summit in Riyadh, his first such comments since the killing earlier this month of The Washington Post columnist Jamal Khashoggi at the Saudi Consulate in Istanbul.

Prince Mohammed bin Salman's anticipated remarks alongside other Arab leaders at the Future Investment Initiative summit in Riyadh come as the event, which was created by the royal, has been overshadowed by Khashoggi's slaying and the international outrage over it.

International business leaders, officials and others have pulled out of the summit, and the event's first day saw several speakers acknowledge the killing of the Saudi writer whose columns criticised the prince's campaign of arrests and governance.

Turkish officials say Khashoggi was killed on Oct 2 by a 15-man Saudi hit squad that included a member of Prince Mohammed's entourage on overseas trips.

Saudi Arabia has suggested, without offering evidence, that the team went rogue.

However, no major decision in the kingdom is made without the approval of the ruling Al Saud family.

Turkish President Recep Tayyip Erdogan kept up his pressure on Wednesday.

"We are determined not to allow the murder to be covered up and for those responsible from the person who gave the order to those who executed it not to escape justice," he said in the capital, Ankara.

President Donald Trump, meanwhile, continued to criticise the kingdom over Khashoggi's killing.

"The cover-up was horrible. The execution was horrible," Trump told journalists on Tuesday night at the White House.

"But there should have never been an execution or a cover-up because it should have never happened."

Trump later was asked about Prince Mohammed in an Oval Office interview with The Wall Street Journal.

"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," Trump told the newspaper.

Shortly after Trump's remarks, United States Secretary of State Mike Pompeo announced that the US was revoking the visas of some Saudi officials implicated in Khashoggi's death.

The visa revocations are the Trump administration's first punitive measures against the Saudis, who are seen as key allies in US efforts to isolate Iran, since Khashoggi disappeared.

Trump meanwhile has been criticising Saudi Arabia and Organisation of Petroleum Exporting Countries (Opec) over high oil prices, calling for a production increase to drop gasoline prices ahead of America's midterm elections.

The foreign ministers of the G7 group of nations said Saudi Arabia should conduct a credible investigation, "in full collaboration with the Turkish authorities".

On Tuesday, the first day of the summit in Riyadh, the crown prince sat alongside King Abdullah II of Jordan during an afternoon session.

Prince Mohammed also looked at some promotional booths outside the main hall as an excited crowd of mostly young Saudi men recorded the encounter on their phones.

At one summit session, Saudi Energy Minister Khalid Al-Falih described Khashoggi's slaying as "abhorrent".

"As we all know, these are difficult days for us in the kingdom of Saudi Arabia," he said.

"Nobody in the kingdom can justify it or explain it. From the leadership on down, we're very upset of what has happened."

The presence of Jordan's king, as well as Pakistani Prime Minister Imran Khan, likely comes as an acknowledgment of the amount of financial support the kingdom offers the two nations.

Lebanese Prime Minister Saad Hariri, who many believe was forced by Prince Mohammed to resign from his position during a visit to the kingdom last year, will also speak on Wednesday at the conference.

Hariri's appearance was scheduled together with that of Prince Mohammed and Bahrain's crown prince.

Pakistan said Saudi Arabia will provide a $6 billion package of loans and deferred payments in an effort to resuscitate Islamabad's flagging economy, struggling under the weight of a whopping $18 billion deficit.

That deal came on the sidelines of the conference on Tuesday.

Pakistan also is seeking a loan from the International Monetary Fund(IMF).

On Tuesday, coinciding with the start of the conference, Erdogan gave a speech to parliament, largely confirming reports and leaks from anonymous officials in past days.

Erdogan said 15 Saudi officials arrived in Istanbul shortly before Khashoggi's death and that a man, apparently dressed in the writer's clothes, acted as a possible decoy by walking out of the consulate on the day of the disappearance.

Turkish investigators have inspected a car belonging to the consulate and found three suitcases, a laptop computer and clothes inside, state television TRT reported.

Authorities discovered the car at an underground garage on Monday.

In Riyadh on Tuesday, King Salman and Prince Mohammed received Khashoggi's son, Salah, and his brother, Sahel, at the Yamama Palace, where the royals expressed their condolences.

A friend of the Khashoggi family told The Associated Press that Salah has been under a travel ban since last year.

The individual spoke on condition of anonymity, fearing reprisal.

Manal Al-Sharif, a Saudi women's rights activist and a friend of Khashoggi, said he "was really assassinated for being outspoken".

"This is a new level the Saudi government is reaching," she said on Wednesday, adding that people inside the kingdom "are so afraid to speak up".

Al-Sharif, who was jailed in Saudi Arabia after getting behind a wheel before the kingdom's ban on women driving was lifted this year, spoke in Denmark where she was promoting her book 'Daring to Drive: A Saudi Woman's Awakening'.