Turkish officials are believed to have an audio recording of the alleged killing of journalist Jamal Khashoggi from the Apple Watch he wore when he walked into the Saudi Consulate in Istanbul over a week ago.

The new claim was published by the Sabah newspaper, through which Turkish security officials have leaked much information about the case.

Pressure is growing on Saudi Arabia to explain what happened to Khashoggi, who has written critically about Saudi Crown Prince Mohammed bin Salman.

President Trump said yesterday the US would inflict "severe punishment" if Saudi Arabia was behind his disappearance. He told CBS: "We're going to get to the bottom of it."

The kingdom has maintained the allegations against it are "baseless", though an official early on Saturday acknowledged for the first time some believe Khashoggi was killed by the kingdom.

Authorities recovered the audio from Mr Khashoggi's iPhone and his iCloud account, according to the report in Sabah newspaper. The journalist had given his phones to his fiancee before entering the consulate.

An Apple Watch can record audio and sync it with an iPhone and an iCloud.

Turkish officials say they believe a 15-member Saudi "assassination squad" killed Khashoggi at the consulate.

Turkey has yet to publish any evidence of him being killed, though surveillance footage around the consulate shows a convoy of vehicles with diplomatic plates leaving the Saudi Consulate for the consul's home in Istanbul a little under two hours after Khashoggi's arrival.

Saudi Arabia has said it had nothing to do with Khashoggi's disappearance, without explaining or offering evidence of how the writer left the consulate and disappeared into Istanbul while his fiancee waited outside.

A Saudi-owned satellite news channel has begun referring to the 15-man team as "tourists", without providing evidence to support the claim.

Early on Saturday, the state-run Saudi Press Agency published a statement from Saudi Interior Minister Prince Abdulaziz bin Saud again denying the kingdom's involvement.

This time, however, it acknowledged for the first time that Saudi Arabia was accused of killing Khashoggi.

Speaking to CBS, President Donald Trump said there is a lot at stake "and maybe especially so because this man was a reporter."

Trump said the Saudis denied "in every way you can imagine" having anything to do with Khashoggi's disappearance but the country may still be responsible, and an investigation is ongoing. "Could it be them? Yes," the president said.

When asked what the US would do if Saudi Arabia is found to be behind the disappearance, Trump wouldn't commit to sanctions. "I'll tell you what I don't want to do," Trump said, "I don't want to hurt jobs. I don't want to lose an order like that. And you know what, there are other ways of punishing."

When asked what was at stake, Trump said: "There's a lot at stake. And, maybe especially so because this man was a reporter. There's something, you'll be surprised to hear me say that, there's something really terrible and disgusting about that if that was the case.

“So, we’re going to have to see. We’re going to get to the bottom of it and there will be severe punishments.”