The United Nations has condemned prosecutors in Saudia Arabia for handing down five death sentences over the killing of Washington Post columnist Jamal Khashoggi.

UN investigator Agnes Callamard said the verdict from the trial was a “mockery” of justice, while Mr Khashoggi’s fiancee has vowed to never forget the people  “trying to cover up” the murder.

“The masterminds not only walk free, they have barely been touched by the investigation and the trial,” Ms Callamard posted to Twitter.

The Saudi court sentenced three more perpetrators to jail for a total of 24 years and acquitted another trio of suspects. The trial had been carried out in near-total secrecy. 

Hatice Cengiz, the fiancee of murdered Washington Post journalist Jamal Khashoggi. Photo: Getty

The investigation concluded that the murder of Mr Khashoggi, a prominent critic of the Saudi government, in October last year “was not premeditated at the start of this mission”.

Rather, Saudi deputy prosecutor Shalaan al-Shalaan said the killing which saw the 59-year-old’s body dismembered, occurred in the heat of the moment.

The court said the verdicts could be appealed.

While rights groups lambasted their decision, Mr Khashoggi’s son said on Monday his family had achieved justice.

“Today we have been granted justice as the children of the deceased, God willing, Jamal Khashoggi. 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 tweeted.

Mr Khashoggi’s fiancee wrote on Tuesday morning that the family would not forget those who tried to “cover up” the murder.

As we will never forget you, we will not forget your murderers, and also those who are trying to cover up your murder! مثلما لن ننساك و لن ننسى قاتليك و أيضا من يسعى لغطاء جريمة القتل #جمال_خاشقجي #JamalKhashoggi pic.twitter.com/GFcB9akOu0

— Hatice Cengiz خديجة (@mercan_resifi) December 23, 2019

Mr Khashoggi was a US resident and hard critic of Saudi Crown Prince Mohammed bin Salman.

He was last seen at the Saudi consulate in Istanbul on October 2, 2018, where he had gone to obtain documents for his impending wedding.

Mohammed bin Salman, Saudi Crown Prince, at the G20 Summit in 2018. Photo: Getty

The crown prince drew international condemnation for the killing because several Saudi agents involved worked directly for him.

The kingdom denies that Prince Mohammed bin Salman had any involvement or knowledge of the operation.

Former high-profile Saudi royal adviser al-Qahtani was investigated but not charged and released. https://t.co/hSojtFlM4I

— Al Jazeera English (@AJEnglish) December 23, 2019

State TV also reported the Saudi attorney general’s investigation showed that the crown prince’s former top adviser, Saud al-Qahtani, had no proven involvement in the killing.

Al-Qahtani, however, has been sanctioned by the United States for his alleged role in the operation.

The court also ruled that the Saudi consul-general in Istanbul at the time, Mohammed al-Otaibi, was not guilty.

He was released from prison after the verdicts were announced, according to state TV.

After holding nine sessions, the trial concluded that there was no previous intent by those found guilty to murder, according to state TV.

The trials of the accused were carried out in near total secrecy, though a handful of diplomats, including from Turkey, as well as members of Khashoggi’s family were allowed to attend the sessions.

Washington Post columnist Jamal Khashoggi was murdered in October 2018.

The killing had shocked the world and drawn condemnation from the international community, including the United Nations.

Khashoggi had walked into his country’s consulate in Istanbul on that morning in October 2018 to collect documents that would allow him to wed his Turkish fiancee, Hatice Cengiz, who waited for him outside.

He never walked out.

Agnes Callamard, a UN special rapporteur who authored an inquiry into Khashoggi’s killing, later said the search for justice must not be left to the Saudi judicial system, which is “so vulnerable to political interference”.

President Donald Trump condemned the killing, and his administration sanctioned 17 Saudis suspected of being involved, though not the crown prince.

Mr Trump, however, has steadfastly resisted calls by members of his own party for a tougher response and has defended maintaining good relations with Saudi Arabia, framing its importance as a major buyer of US military equipment and weapons and saying this creates American jobs.

Meanwhile, numerous critics of the Saudi crown prince remained imprisoned and face trial for their acts of dissent.

-with AAP