Warning: Graphic content

A source has claimed Jamal Khashoggi was butchered alive in a brutal seven-minute execution after reportedly hearing audio of the missing journalist's horrifying last moments.

The anonymous source, who claimed to have heard a recording captured on Khashoggi's Apple watch, said in an interview with Middle East Eye that Khashoggi was killed after entering the Saudi consulate in Istanbul on October 2.

According to the source, the tape reveals that Khashoggi was dragged from the Consul General's office to a table next door in a study where he was surgically dismembered.

Audio recording is said to have captured the missing journalist's dying screams before he was "injected with an unknown drug" and fell silent.

"There was no attempt to interrogate him. They had come to kill him," the source said.

The source also claimed that Khashoggi's screams had been heard by witnesses downstairs as he was cut into pieces on the desk.

The news comes as Turkish police said earlier Tuesday they now have evidence missing journalist Jamal Khashoggi was killed and cut into pieces inside the Saudi consulate in Istanbul.

The discovery was reportedly made during the nine-hour overnight search, after which crime scene investigators have been "looking into toxic materials".

Just hours before investigators were due to search through the residence of Saudi consul Mohammad al-Otaibi in Istanbul, he left Turkey on a commercial flight.

Meanwhile President Trump said Tuesday that he has spoken to the crown prince of Saudi Arabia and the kingdom steadfastly denies any involvement in Khashoggi's suspected murder.

Trump said that Saudi Crown Prince Mohammed bin Salman "totally denied" knowledge of the journalist's disappearance in a Tuesday afternoon phone call.

A Turkish official confirmed to CNN late on Tuesday that police believe Khashoggi was brutally dismembered, after a previous report in the New York Times shortly after his disappearance made similar claims.

Earlier on Tuesday Police claimed to have found "certain evidence" that Khashoggi was killed inside the consulate, before the Saudi consul made a sudden departure from a Turkish airport.

Just spoke with the Crown Prince of Saudi Arabia who totally denied any knowledge of what took place in their Turkish Consulate. He was with Secretary of State Mike Pompeo...

— Donald J. Trump (@realDonaldTrump) October 16, 2018

In tweets, Trump said that the crown prince, who goes by the initials MBS, reiterated his father's denial of Saudi Arabia's alleged role in the affair during the call that followed a dinner between Secretary of State Mike Pompeo and the Saudi leader in Riyadh.

He said in an interview just before the call that he was waiting to hear from the crown prince to pass judgement. "If they knew about it, that would be bad," he said, reflecting on the matter to Fox Business.

The president declined to weigh in, once again, on the believability of the claims that Turkey says are bogus.

President Recep Tayyip Erdogan said today he hoped a "reasonable opinion" would be reached as soon as possible, clarifying what happened when the Saudi journalist entered his country's consulate in Istanbul two weeks ago.

Seeking to defuse the crisis over Khashoggi's disappearance, US Secretary of State Mike Pompeo today arrived in Riyadh to meet with King Salman and Crown Prince Mohammed bin Salman of Saudi Arabia.

Just spoke with the Crown Prince of Saudi Arabia who totally denied any knowledge of what took place in their Turkish Consulate. He was with Secretary of State Mike Pompeo...

— Donald J. Trump (@realDonaldTrump) October 16, 2018

"We are strong and old allies. We face our challenges together," the crown prince said as he warmly welcomed Pompeo at the palace before sitting down for talks and a subsequent dinner.

Pompeo will travel straight to Turkey to hold talks with his Turkish counterpart Mevlut Cavusoglu on Wednesday, Ankara said in a statement.

This follows reports last night that Saudi Arabia is preparing to admit that the 59-year-old Washington Post journalist was killed inside the Istanbul consulate - but that it was an accident.

CNN claimed that the Saudi Arabian government is preparing a report which will say Khashoggi was supposed to be taken to the kingdom but died in the consulate during interrogation.

CNN said that two sources had leaked the report, "which could change" and is "still being prepared".

The Saudis will likely claim the alleged murder was carried out "without clearance and transparency" and that the "hit squad" of 15 Saudi assassins will be punished, the sources said.

Previously, sources in British intelligence have been quoted by Reuters as saying they believe there had been an attempt to drug Khashoggi inside the consulate that culminated in an overdose.

Turkish officials have said that authorities have an audio recording indicating that Khashoggi was killed in the consulate, and have shared evidence with countries including Saudi Arabia and the United States.

Meanwhile, UN human rights chief Michelle Bachelet called for the immediate and "absolute" lifting of diplomatic immunity enjoyed by any officials or premises in the Khashoggi investigation.

Bachelet said the "inviolability or immunity" of people or premises granted under the 1963 Vienna Convention on Consular Relations "should be waived immediately."

She said Tuesday the "onus is on the Saudi authorities" to reveal what happened, and insisted "no further obstacles" should be placed in the way of a quick, thorough, impartial and transparent investigation.

Bachelet stopped short of calling for an international investigation.

Last night, a team of Turkish and Saudi officials entered the Saudi consulate in Istanbul for a joint inspection two weeks after Khashoggi went missing.

At least a dozen officials arrived in unmarked police cars at 6pm local time and mingled outside the building before filing inside.

The inspection was agreed after Saudi King Salman and Turkish President Erdogan spoke yesterday for the first time since Turkey accused the Saudis of killing and dismembering Khashoggi who criticised the Saudi crown prince.

Just hours before the forensic inspection, the Saudis let in a team of cleaners armed with several large mops and buckets.

Searching another country's consulate - which is considered foreign soil under the Vienna Convention - is an extraordinary measure which reflects the gravity of the diplomatic crisis.

A Turkish official yesterday claimed the consulate walls have been repainted since the alleged murder and said the Turks don't trust the Saudis not to obfuscate the investigation, reported the Middle Eastern Eye.

On Monday, US Trump speculated "rogue killers" were to blame after revealing the Saudi king denied any murder plot during a phone conversation between the pair last night.

Speaking to reporters in the White House, Trump said King Salman's denial "could not have been stronger."

"He said it very strongly," Trump said when pressed to say whether he believed the Saudi king.

He added: "It sounded to me like maybe these could have been rogue killers. Who knows?

"I heard that [CNN] report, but nobody knows if it's an official report," he said in Georgia. "So far, it's just a rumour of a report coming out."

President Trump has previously said he does not want to halt a proposed $110 billion arms sale to Saudi Arabia - as some in Congress have suggested - because it would harm the US economically.

However, on Monday, White House economic adviser Larry Kudlow warned that the United States would "take stern action with the Saudis if necessary."

Khashoggi, who was notoriously critical of Saudi Arabia's new Crown Prince, entered the consulate on 2 October to get documents to marry his Turkish fiancee - but has not been seen since.