The trial of 11 men accused of murdering the Saudi Arabian journalist Jamal Khashoggi, which started in the kingdom on Thursday, has been slammed by the United Nations.

The UN’s human rights office said it had no way of assessing the fairness of the trial – in which five of the suspects are facing the death penalty.

Spokeswoman Ravina Shamdasani called for an independent investigation into the killing and demanded “international involvement”.

The US has also piled on the pressure, with a senior official saying Saudi Arabia’s investigation still lacks full credibility and accountability.

Mr Khashoggi – a Washington Post columnist who was critical of the current Saudi leadership – was killed while visiting the country’s embassy in Istanbul last October. The 59-year-old was living in self-imposed exile in the US at the time.

Leaked intelligence reports suggest he was tortured – including having his fingers cut off – before he was killed. His body was said to have been dissolved in acid.

The slaying caused revulsion around the world as Turkish investigators revealed a team of 15 Saudis had been flown into Istanbul for 24 hours with the apparent intention of killing Mr Khashoggi during a prearranged appointment at the embassy.

Moments after he died one of the men was recorded phoning a number in the Saudi capital, Riyadh, and saying: “The thing is done, it’s done.”

After changing its response to the incident several times, the Saudi government eventually said the killing had been a “rogue” operation.

It denied the kingdom’s de fact ruler, Crown Prince Mohammed bin Salman, had ordered the murder. The CIA has since said it is highly unlikely the killing could have happened without him being aware of it.

Following the UN’s assessment of the trial on Friday, US officials expressed their own doubts about its plausibility.

“I don’t think, from our point of view, that the narrative emerging from the Saudis throughout the legal process has yet hit that threshold of credibility and accountability,” an official told journalists.

Rights groups have also called for an independent investigation into the killing.

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“Given the possible involvement of Saudi authorities in Khashoggi’s murder and the lack of independence of Saudi Arabia’s criminal justice system, the impartiality of any investigation and trial would be in question,” Samah Hadid, a Middle East director at Amnesty International, said on Thursday. “This is why a UN-led and independent investigation is needed into the murder.”