As Saudi authorities attempted to persuade the world that Washington Post columnist Jamal Khashoggi died during a brawl inside the country’s Istanbul consulate, Australia has announced it will boycott an upcoming financial summit in the oil-rich desert kingdom.

it is “no longer appropriate” to attend the Future Investment Initiative in Riyadh, Foreign Affairs Minister Marise Payne announced on Saturday.

The moves amid revelations in the New York Times that Saudi Arabia unleashed an army Twitter trolls to villify the slain journalist and other opponents of the Saudi regime.

The covert bid to sway public opinion via social media, included a so-called troll farm based in Riyadh.

The paper also reported that a suspected Saudi operative was working within Twitter to monitor user accounts.

Twitter declined to comment. A representative from the Saudi embassy in Washington did not respond to a request for comment.

Senator Payne said she had conveyed to the Saudi Ambassador to Australia the government’s “serious concern” for Mr Khashoggi’s welfare and also directed the Australian ambassador in Riyadh to raise the matter.

“We strongly urge the Saudi authorities to conduct a thorough investigation and to co-operate fully with their Turkish counterparts in ascertaining the full circumstances of Mr Khashoggi’s death,” she said.

“We call on Saudi Arabia to hold all those responsible to account for this egregious act against one of its citizens.”

The Saudi public prosecutor said in a statement a fight broke out between Khashoggi and people who met him inside the consulate, after he entered the building on August 2 seeking documents for his planned marriage.

“The investigations are still underway, and 18 Saudi nationals have been arrested,” the statement on state media said, adding that royal court adviser Saud al-Qahtani and deputy intelligence chief Ahmed Asiri had been fired from their positions.

Two other Saudi officials have already been sacked, Mr Asiri confirmed.

Saturday’s comments mark the first time since Khashoggi, a strong critic of Crown Prince Mohammed bin Salman, went missing that the Saudis have admitted to his death.

Speaking from Luke Airforce Base in Arizona on Friday (local time), US President Donald Trump said the arrest of the 18 Saudis was an “important first step”.

“Saudi Arabia has been a great ally but what happened is unacceptable,” he said.

When asked about his confidence in the explanation from the Saudis that Khashoggi died after a fight, he said: “I do. I do.”

“Again, it’s early. We haven’t finished our review, or investigation. But I think it’s a very important first step and it happened sooner than people thought it would happen.”

Later on Saturday, the official English Twitter account of the Ministry  of Foreign Affairs of the Kingdom of Saudi Arabia said it “expresses its deep regret at the painful outcome and stresses the commitment of the authorities in the Kingdom to bring the facts to the public”.

Turkish police officers leave after searching evidence at the Saudi Arabian Consulate in Istanbul on Thursday. Photo: Getty

“The investigations are still ongoing with the (18) detainees of Saudi nationality.

“The investigation showed the death of Jamal Khashoggi during a fight in the Consulate,” it posted.

US citizen Khashoggi’s sudden disappearance strained relations between Saudi Arabia and Western allies, with President Trump ramping up his rhetoric earlier in the week demanding answers otherwise repercussions against Saudi Arabia would be “very severe”.

On Thursday (local time), Turkish police intensified their investigation into Khashoggi’s disappearance after DNA samples taken from the Saudi consulate, including soil and water, led them to search rural and forest areas outside Istanbul.

Turkish police extended their search to Belgrad Forest near Istanbul on October 19. Photo: AAP

“The investigations led to some suspicion that his remains may be in the city of Yalova and the Belgrad forest, police have been searching these areas,” one of the officials said.

A “farm house or villa” may have been used for the disposal of his remains, the official said.

Specifically, the Belgrad Forest is located just a 15-minute drive from Istanbul and is a popular tourist location for families. A second rural location near the city of Yalova, a 90-kilometre drive south of Istanbul on the Sea of Marmara, is a weekend retreat for Istanbul locals, famous for its thermal hot springs and hiking.

Earlier in the week, Mr Trump said he presumed Khashoggi is dead, sending Secretary of State Mike Pompeo to Saudi Arabia for urgent talks to get to the bottom of what exactly happened.

“Mike Pompeo returned last night from Saudi Arabia and Turkey. I met with him this morning wherein the Saudi situation was discussed in great detail,” Mr Trump posted on Twitter on Thursday (local time).

Authorities widened the geographic focus of the search after tracking the routes and stops of cars that left the Saudi consulate and the consul’s residence on October 2, the day Khashoggi was last seen, the senior officials said.

Mr Trump said it was too early to say what the consequences for the incident might be, but that the US Congress would be involved in determining the American response.

Asked whether Saudi sanctions were one of the measures he was considering, Mr Trump said: “Could be, could be,” though he provided no details.

“We’re going to find out who knew what, when and where. And we’ll figure it out,” he added.

Khashoggi’s disappearance has tarnished the crown prince’s reputation and deepened questions about his leadership, prompting Saudi King Salman to intervene, five sources with links to the Saudi royal family told Reuters.

Saudi Arabia had earlier denied allegations by Turkish officials who have said the Washington Post columnist was killed inside the building and his body removed, describing them as “baseless” allegations.

-with agencies