RIYADH (REUTERS) - Saudi Arabia on Monday (Dec 23) sentenced five people to death and three to jail over the murder of Saudi journalist Jamal Khashoggi, but a UN investigator accused it of making a "mockery" of justice by allowing the masterminds of last year's killing to go free.

A Saudi court rejected the findings of a UN inquiry by ruling that the killing was not premeditated, but carried out "at the spur of the moment".

Saudi Deputy Public Prosecutor and spokesman Shalaan al-Shalaan also said the court dismissed charges against three of the 11 people tried, finding them not guilty.

A senior official of the Trump administration, which critics say has been too soft on Saudi Arabia over the killing of Khashoggi, a US resident and critic of the kingdom's de facto ruler Crown Prince Mohammed bin Salman, called the verdicts "an important step" in holding those responsible accountable.

However, a source familiar with US intelligence assessments said key US government agencies rejected the validity of the proceedings and Central Intelligence Agency experts still believed the crown prince personally ordered, or at least approved of, the killing.

The source said the five men condemned to death were essentially footsoldiers in the killing, while two senior security officials acquitted played a more significant role.

A Saudi prosecutor said there was no evidence connecting one of those senior officials, Saud al-Qahtani, to the killing and the court dismissed charges against Ahmed al-Asiri, a former deputy intelligence chief.

Khashoggi was a US resident and critic of Saudi Crown Prince Mohammed bin Salman, the kingdom's de facto ruler. He was last seen at the Saudi consulate in Istanbul on Oct 2, 2018, where he had gone to obtain documents for his impending wedding.

His body was reportedly dismembered and removed from the building, and his remains have not been found.

Eleven Saudi suspects were put on trial over his death in secretive proceedings in the capital Riyadh.

Khashoggi's murder caused a global uproar, tarnishing the crown prince's image. The CIA and some Western governments have said they believe Prince Mohammed, also known as MbS, ordered the killing.

Saudi officials say he had no role, though in September the Prince for the first time indicated some personal accountability, saying "it happened under my watch".

Agnes Callamard, the UN special rapporteur for extrajudicial summary or arbitrary executions, lambasted the trial verdict as a "mockery" of justice.

"The hit-men are guilty, sentenced to death. The masterminds not only walk free, they have barely been touched by the investigation and the trial," she said on Twitter.

The UN-led inquiry reported in February that the evidence pointed to "a brutal and premeditated killing, planned and perpetrated" by Saudi officials.

TWO SENIOR FIGURES FREED AFTER PROBE

Last November, the Saudi prosecutor said Saud al-Qahtani, a former high-profile Saudi royal adviser, had discussed Khashoggi's activities before he entered the Saudi consulate with the team which went on to kill him.

The prosecutor had said Qahtani acted in coordination with deputy intelligence chief Ahmed al-Asiri, who he said had ordered Khashoggi's repatriation from Turkey and that the lead negotiator on the ground then decided to kill him.

Both men were dismissed from their positions but while Asiri was tried, Qahtani was not.

On Monday, Shalaan said Asiri has been tried and released due to insufficient evidence, and Qahtani had been investigated but was not charged and had been released.

Shalaan also said the Saudi consul-general to Turkey at the time, Mohammed al-Otaibi, had been freed after Turkish witnesses said Otaibi had been with them on the day of the crime. Two weeks ago, the United States barred Otaibi from entering the country.

Sources familiar with the matter told Reuters last year that Maher Mutreb, the lead negotiator, and Salah al-Tubaigy, a forensic expert specialising in autopsies, were also on trial for the murder and could face the death penalty.

On Monday, Shalaan said that when the Saudi team that entered the consulate saw it would not be possible to transfer Khashoggi to a safe place to continue negotiating, they decided to kill him.

"It was agreed, in consultation between the head of the negotiating team and the culprits, to kill Jamal Khashoggi inside the consulate," Shalaan said in response to questions from journalists.

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Noting the verdict, UN Secretary-General Antonio Guterres stressed the need for "an independent and impartial investigation into the murder", a spokesman said.

Turkey said on Monday the trial outcome was far from serving justice and repeated its call for better Saudi judicial cooperation.

"The fact that important issues like the location of the late Khashoggi's body, the identification of the instigators and, if there are any, the local co-operators, are still in the dark is a fundamental shortcoming to justice being served and accountability," Foreign Ministry spokesman Hami Aksoy said.

Callamard, the UN rapporteur, criticised the trial for having been held behind closed doors, saying that the conditions in international law for doing so had not been met.

Human rights group Amnesty International also criticised the closed trial, branding the verdict a "whitewash" that it said failed to address the Saudi authorities' involvement or the location of Khashoggi's remains.

However, one of Khashoggi's sons - who weeks after the murder appealed publicly for the return of his father's body - said the verdicts had been fair to his children.

"We affirm our confidence in the Saudi judiciary at all levels, that it has been fair to us and that justice has been achieved," Salah Khashoggi said on Twitter.

On Twitter, a favourite platform for government supporters, many Saudis hailed Qahtani's exoneration, sharing pictures and describing him as a "loyal defender of the kingdom".

Last week, Twitter removed 6,000 accounts it said appeared to be linked "to a significant state-backed information operation" originating in Saudi Arabia.

Riyadh's criminal court pronounced the death penalty on five defendants "for committing and directly participating in the murder of the victim". The three sentenced to prison were given various sentences totalling 24 years "for their role in covering up this crime and violating the law".

Shalaan added the investigations proved there was no "prior enmity" between those convicted and Khashoggi.

The verdicts can still be appealed.