Saudi Arabia’s crown prince told a senior aide he would go after Jamal Khashoggi “with a bullet” a year before the dissident journalist was killed inside the kingdom’s Istanbul consulate, according to a US media report.

US intelligence understood that Mohammed bin Salman, the country’s 33-year-old de facto ruler, was ready to kill the journalist, although he may not have literally meant he planned to shoot him, according to the New York Times ($).

After initially denying any knowledge of Khashoggi’s disappearance, the kingdom has acknowledged that a team killed him inside the diplomatic mission but described it as a rogue operation that did not involve the crown prince.

The Saudi minister of state for foreign affairs, Adel al-Jubeir, on Friday said Prince Mohammed did not order Khashoggi’s murder but could not comment on the New York Times’ report.

The conversation between the prince and his aides was intercepted by US intelligence agencies, as part of routine efforts by the National Security Agency and other security services to capture and store the communications of global leaders, including allied ones, the newspaper reported.

It was only recently transcribed, however, because of mounting efforts by US intelligence to find conclusive proof linking the prince to the killing.

The conversation took place between the crown prince and an aide, Turki Aldakhil, in September 2017 – about 13 months before the killing, the paper said.

The prince said that if Khashoggi could not be enticed to return to Saudi Arabia, then he should be brought back by force. If neither of those methods worked, then he would go after Khashoggi “with a bullet”, the paper reported.

The report came after a UN human rights expert looking into the case said the Saudi regime “seriously curtailed and undermined” the Turkish investigation into the murder of Khashoggi.

Agnès Callamard, a UN special rapporteur on extrajudicial killings, said the Saudi dissident and Washington Post columnist was the victim of a “brutal, premeditated killing planned and perpetrated by officials of the state of Saudi Arabia”.

Khashoggi was lured into the Saudi consulate in Istanbul on the promise of being given documents that would help him remarry. Inside he was suffocated and then his body was dismembered, according to a Turkish investigation.

In a preliminary report, Callamard said she had heard “parts of the chilling and gruesome audio material obtained and retained by the Turkish intelligence agency”.

She said: “Woefully inadequate time and access was granted to Turkish investigators to conduct a professional and effective crime-scene examination and search required by international standards for investigation.”

Callamard, a French human rights expert, is due to deliver a final report to the UN human rights council in June. On Thursday she provided an assessment of her visit to Turkey to pursue the investigation, from 28 January and 3 February.

She said Saudi killers had exploited diplomatic immunity to carry out the murder.

“Guarantees of immunity were never intended to facilitate the commission of a crime and exonerate its authors of their criminal responsibility or to conceal a violation of the right to life,” Callamard said. “The circumstances of the killing and the response by state representatives in its aftermath may be described as ‘immunity for impunity’.”

Callamard also criticised the kingdom for carrying out a secretive second trial on 31 January of 11 men indicted by the Saudi authorities over Khashoggi’s murder.

There are major concerns about the fairness of the Saudi proceedings, she said, adding that her team had not yet received permission to visit Riyadh.

“Given the importance of the case, we should be expecting a greater presence of representatives of the media, of civil society, of a range of other governments, not just those hand-picked by the Saudi authorities,” the special rapporteur said.

US intelligence chiefs have told Congress that Prince Mohammed almost certainly ordered the killing or was aware of it, even though Donald Trump and his secretary of state, Mike Pompeo, have insisted the evidence is incomplete and investigations would continue. Pompeo met al-Jubeir in Washington on Thursday.

Khashoggi’s fiancee, Hatice Cengiz, said on Friday that she would welcome a change in Trump’s approach to the killing, and may visit the US in March, having previously rejected an invitation from the president. “The [US] congress is renewed, the members are changed. I am hopeful. There is still hope. I believe the new congress would follow this issue closer,” Cengiz said.

Riyadh has denied the prince was involved. The Saudi public prosecutor has charged 11 men with the murder, saying last month that he would seek the death penalty for five of them. The defendants’ names and those of their lawyers have not been revealed, and it is also unclear if the suspects are being detained between hearings or where they are being held.

Turkey has repeatedly asked for the extradition of the Saudi suspects for trial in Istanbul, requests denied by Riyadh, which says they will face justice within the Saudi court system.

Fahrettin Altun, communications director for the Turkish president, said: “The world is watching.”