The lack of action by the United Nations after the brutal slaying of the dissident Saudi journalist Jamal Khashoggi raises questions about the organisation's credibility, his fiancee said.

It was high time for the UN -- and individual countries -- to take action against those responsible, Hatice Cengiz added.

A UN rights expert who wrote a damning report on the killing also regretted the "disappointing" response from UN chief Antonio Guterres.

More than 17 months after his murder, Hatice Cengiz was at the UN in Geneva this week to press her case for justice for Khashoggi, a harsh critic of Saudi Crown Prince Mohammed bin Salman.

Khashoggi, a Washington Post contributor and US resident, was killed on October 2, 2018 while at Saudi Arabia's consulate in Istanbul to obtain paperwork ahead of his wedding to Cengiz.

Turkish officials say a 15-man Saudi squad strangled him and cut his body into pieces. His remains were never found. Riyadh insists he was killed in a "rogue" operation.

The killing brought international condemnation, but Cengiz told reporters Monday that the lack of follow-up action was "causing me and others to question the validity of the UN".

Hatice Cengiz has been campaigning for a tougher response from the international community
Hatice Cengiz has been campaigning for a tougher response from the international community AFP / SAUL LOEB

Speaking through a translator, the Turkish researcher said it was high time for the UN to "assert themselves as the body that has a right to bring sanctions... to punish those who are guilty".

She urged world nations to launch a proper investigation and stop putting ties with Saudi Arabia before their values.

Many European nations were "really uncomfortable" with the "hideous crime", said Cergiz.

"I'm here especially to encourage the countries that (do not wish to) keep political and economic interests ahead of everything else," she said.

"Although I personally was most affected by this, the whole world knew about it."

UN United Nations Special Rapporteur Agnes Callamard wants Turkey to put the suspects on trial in absentia
UN United Nations Special Rapporteur Agnes Callamard wants Turkey to put the suspects on trial in absentia AFP / Aris Oikonomou

Agnes Callamard, the UN special rapporteur on extrajudicial, summary or arbitrary executions, echoed her position Monday. In June last year, she released a damning report that found "credible evidence" linking the Saudi crown prince to the killing.

The independent rights expert, who does not speak for the UN but who reports her findings to it, urged the global body to initiate an international criminal probe.

"What's happening right now is a major indictment of the capacity of the international community to hold governments to account," she told AFP.

Callamard said world nations "should not be held hostage of the Saudi unwillingness and inability to meet their obligations".

She urged UN Secretary General Antonio Guterres to be more bold, saying the institution was "weak" on dealing with targeted killings.

"It is disappointing that the secretary-general has not done or said much about the killing," the expert said.

"The UN... is far too politicised to conduct a proper inquiry into a targeted killing and it has no easily accessible mechanism to do so."

Callamard called on Turkey to put the suspects on trial in absentia about the case and said countries attending the G20 in Riyadh in November would be "almost in acquiescence with the situation".

Despite a torrent of international condemnation over the murder of Khashoggi, Saudi Arabia's de facto ruler Prince Mohammed appears to have strengthened his position.

Saudi authorities have detained three princes including King Salman's brother and nephew for allegedly plotting a coup, three sources told AFP on Saturday, signalling Prince Mohammed's tightening grip on power.

The detentions cast aside the last vestiges of potential opposition to the crown prince.

"What do you expect after someone has killed a person in a consulate?" said Cengiz.

"We have sent them a message that they can get away with this."