Agnes Callamard, a UN special rapporteur who concluded that Khashoggi's death at the Saudi consulate in Istanbul in October was "an extrajudicial execution" by the Gulf kingdom, criticised the United States over its inaction.

"(It) has the jurisdiction or at least the interest to take action," she told a London conference hosted by human rights groups on the killing of the Saudi-born US resident.

"Silence is not an option. Speaking up is required but not enough. We have to act," Callamard said.

The UN rapporteur on extrajudicial, summary or arbitrary executions said Washington could act "either through an FBI investigation (or) a civil law investigation... (or) the declassification of CIA and other materials".

"All of those things I believe can be done and should be done."

Khashoggi, a Washington Post contributor, was killed last October by Saudi agents while at Saudi Arabia's consulate in Istanbul to obtain paperwork.

His dismembered body has not been found.

Callamard launched her investigation in January and last month released an 101-page report that found "credible evidence" linking Saudi Crown Prince Mohammed bin Salman to the murder and an attempted cover up.

It said further investigation and financial sanctions were warranted.

The rapporteur noted she had received no cooperation from Riyadh and minimal help from the United States.

"The US was not at the top of the cooperation chain," Callamard said.

"They did the minimum to keep them within the remit of what is expected from a Western government."

Callamard added she was not granted any access to the CIA, the US Department of Justice or other Trump officials.

Amid frustration at the global inaction since the publication of her findings on June 19, and Prince Salman being welcomed to a recent G20 meeting in Japan, she said the West risked a "democratic deficit" in not responding to widespread public disgust at the killing.

"That is dangerous... that democratic deficit must be tackled."

Callamard does not speak for the UN but reports her findings to it.

She has called on Secretary General Antonio Guterres to initiate an international criminal investigation into the case.

His office has said he does not have the authority to do this and a member state must initiate such action.

Hatice Cengiz, Khashoggi's Turkish fiancee, appeared alongside Callamard at the London event and echoed her call for justice.

"We ask all European countries and especially the UK to take this report more seriously," she said.

"It's too dangerous to behave as if nothing has happened."