A top Turkish official, presidential adviser Yasin Aktay, has said he believes Jamal Khashoggi's body was dissolved in acid after being cut up.
The "only logical conclusion", he said, was that those who had killed the Saudi journalist in Istanbul had destroyed his body "to leave no trace behind".
Khashoggi, a critic of Saudi rulers, was killed inside the country's consulate on 2 October.
No forensic evidence has been provided to prove his body was dissolved.
"The reason they dismembered Khashoggi's body was to dissolve his remains more easily", Mr Aktay told the Hurriyet Daily newspaper.
"Now we see that they did not only dismember his body but also vaporised it."
The claims came as Khashoggi's fiancée, Hatice Cengiz, called on world leaders to "bring the perpetrators to justice", in an editorial for five newspapers, including the Guardian and the Washington Post.
Meanwhile, reports quote Saudi Crown Prince Mohammed bin Salman as telling the US he considered Khashoggi to be a dangerous Islamist.
The reported phone call to the White House came before Saudi Arabia admitted Khashoggi had been killed.
Saudi Arabia has denied the comments were made or that its royal family was involved in the killing, and says it is "determined to find out all the facts".
Istanbul's prosecutor confirmed on Wednesday that the writer had been strangled.
What was said in the crown prince's alleged phone call?
During the call with President Donald Trump's son-in-law Jared Kushner and national security adviser John Bolton, Prince Mohammed said Khashoggi had been a member of the Muslim Brotherhood, a transnational Islamist organisation, the Washington Post reports.
The phone call is reported to have taken place on 9 October, a week after Khashoggi disappeared.
Prince Mohammed also reportedly urged the White House to preserve the US-Saudi alliance.
In a statement to the newspaper, Khashoggi's family denied he had been a member of the Muslim Brotherhood and said the murdered writer had himself denied this repeatedly in recent years.
"Jamal Khashoggi was not a dangerous person in any way possible. To claim otherwise would be ridiculous," the statement said.
What has the investigation found so far?
There is still no consensus on how Khashoggi died. He entered the consulate to sort out documents for his marriage.
But on Wednesday Turkey said he had been strangled immediately after entering the consulate and his body dismembered "in accordance with plans made in advance".
Turkish media had previously quoted sources as saying Turkey had audio recordings proving that Khashoggi had been tortured before being murdered.
Saudi Arabia has changed its account of what happened to Khashoggi.
When he first disappeared, it said Khashoggi had walked out of the building alive. It later admitted he had been murdered, saying the killing was premeditated and a result of a "rogue operation".
It has arrested 18 suspects who, it says, will be prosecuted in Saudi Arabia. Turkey wants the suspects to be extradited.
Turkey has steered away from publicly blaming Saudi Arabia for the killing.
President Recep Tayyip Erdogan spoke to Saudi King Salman last week, and the two agreed to continue co-operating in the investigation.
What do other countries say?
Saudi Arabia has faced a backlash over the death, including from its allies, who have called for answers.
President Trump has said he is "not satisfied" with the Saudi account. However, he also said he was unwilling to sacrifice lucrative arms deals with the country.
Although their US visas have been revoked, Secretary of State Mike Pompeo said it would be a "handful more weeks" before the US knew enough to impose sanctions on individuals involved in Khashoggi's killing.
Mr Pompeo said the US had "deep and long-term strategic relationships" with Saudi Arabia and said "we intend to make sure that those relationships remain intact".
On Wednesday, France's Foreign Minister Jean-Yves Le Drian said Khashoggi's death was a "crime" and "odious".
He said France was not "dependent on our economic relations with Saudi Arabia" and the country would impose sanctions, but no details were given.
UK Foreign Secretary Jeremy Hunt also said it was an appalling act, adding that it had "possibly" given the US and the UK a chance to put new pressure on Saudi Arabia over other issues.
Earlier, the US called for a swift cessation of hostilities in Yemen, where a Saudi-led coalition has been fighting rebels supported by its arch-enemy Iran.
What has his fiancée said?
In an editorial published in five international newspapers, Ms Cengiz said Khashoggi had just bought a house and wished to start a family.
She described the "anguish" she had experienced since his "brutal, barbaric and ruthless" death.
"We must all send a clear message that authoritarian regimes cannot kill journalists ever again."
She called on governments around the world to take action to reveal the truth, accusing the United States of taking a position "devoid of moral foundation."
"If the democracies of the world do not take genuine steps to bring to justice the perpetrators of this brazen, callous act - one that has caused universal outrage among their citizens - what moral authority are they left with?" she asked.