Saudi Arabia’s crown prince, Mohammed bin Salman, has broken a three-week public silence over the killing of Jamal Khashoggi, describing the journalist’s murder as a “heinous crime that cannot be justified”.

Speaking in Riyadh at the Future Investment Initiative conference, nicknamed “Davos in the desert”, the prince said all culprits would be punished, and that “justice would prevail”.

They were his first remarks since the global outcry over the killing on 2 October of the Washington Post columnist at the Saudi consulate in Istanbul, an event that continues to reverberate around the region. Members of the prince’s security staff are among those named by Turkish authorities as having carried out the killing.

Profile Who was Jamal Khashoggi? Show Hide Jamal Khashoggi was one of the Arab world’s most prominent journalists and commentators. He was an outspoken critic of Saudi Arabia who dared to defy Crown Prince Mohammed bin Salman. While living in Saudi Arabia, Khashoggi was told to stop writing or posting on Twitter, where he had more than 1.6 million followers. He moved to the US in June 2017, where he continued to comment on his country both in print and on television. He wrote columns for the Washington Post and the Guardian.

His message struck a nuanced tone in the US, where he tried to acknowledge the reforms undertaken by Bin Salman while also highlighting the flaws. Khashoggi previously had close links with the Saudi royal family, including having served as a media aide to Prince Turki al-Faisal, when the latter was director general of the Saudi intelligence agency. He was also a former editor of the Saudi newspaper al-Watan and had worked with Prince Alwaleed bin Talal, a grandson of the first Saudi king. Khashoggi was murdered in the Saudi Arabian consulate in Istanbul in October 2018. Photograph: Virginia Mayo/AP

Bin Salman announced a restructuring of the kingdom’s national security agencies and said Saudi Arabia and Turkey would work together “to reach results”.

“The incident that happened is very painful, for all Saudis … The incident is not justifiable,” he said. “They will not be able to divide us as long as there is a king called King Salman bin Abdulaziz and a crown prince named Mohammed bin Salman, and a president in Turkey named Erdoğan.”

But in a sign that the Saudis’ attempts to address the issue had failed to convince its western allies, No 10 said Theresa May had called King Salman on Wednesday “to reiterate the UK’s grave concerns about the murder”. A Downing Street spokesman said she had told the king that “the current explanation lacks credibility. She also reiterated that all individuals bearing responsibility for the killing must be properly held to account.” She said suspects would be prevented from entering the UK and any visas would be revoked.

The crown prince’s remarks were made to a full auditorium of onlookers and guests who listened silently as he responded to a question from a moderator. He distanced himself from the 18 people alleged to have killed and dismembered Khashoggi inside the consulate, and appealed to the Turkish president, Recep Tayyip Erdoğan, as a partner. On Tuesday, Erdoğan directly accused some of the “highest ranked” Saudis of ordering Khashoggi’s killing, but avoided mentioning Bin Salman by name.

The prince’s remarks received a short burst of applause and lifted a subdued mood that had heralded his arrival. His entrance had much less fanfare than the inaugural event last year, which had marked the global debut of the 33-year-old heir to the throne.

“He’s not going to address the humungous elephant in the room, I bet,” one conference attendee, Heba Saleh, said before the prince’s appearance. Stunned silence met the first mention of Khashoggi’s name, and a palpable relief followed.

The gruesome killing and its profound implications have cast a pall over the conference, which has been widely boycotted by the international business elite, and partially rescued by regional leaders and last-minute delegations, many of whom have filled seats but pledged little business. American business people are thin on the ground, in stark contrast to 2017, when the halls of the Ritz Carlton conference centre teemed with executives from Fortune 500 companies.

Some delegates did nothing to hide their disdain for both the crime and the subsequent attempts to cover it up. Four of the alleged killers were members of Bin Salman’s security detail, and the remainder came from the Saudi security establishment.

“These idiots have taken us back to the Stone Age,” said one delegate, who refused to be named. “How do I defend this country to anyone any more? The stupidity here is unparalleled.”

Others said attempts to quarantine Bin Salman from an act blamed on his closest personal staff, including his domestic enforcer, Saud al-Qahtani, were doomed to backfire. “It suits many of us to believe this, because the alternative is just too impossible,” said a Saudi businessman who called himself Ahmed Mohammed. “But anyone who has lived here understands the fiction. And his friends outside cannot be expected to look away like us.”

Khaled Mansour, another Saudi businessman, said the country had little option but to fall in behind Bin Salman, and that holding senior leaders to account for Khashoggi’s death would set the country back further. “This is very complicated,” he said. “It is painful for the family and for the people. It is best to live in denial here.”

On Tuesday, Donald Trump, an ally of Bin Salman, said for the first time that the prince could have been involved in the operation to kill the dissident Saudi journalist. “Well, the prince is running things over there, more so at this stage. He’s running things and so if anybody were going to be, it would be him,” he said.

On Wednesday, the Washington Post added to reports in Turkish media that CIA director Gina Haspel had listened to an audio recording said to prove Khashoggi was brutally tortured before his premeditated murder. Haspel flew to Ankara on Monday night to assist Turkey’s investigation into Khashoggi’s death. Four sources familiar with the mission brief told Reuters she had asked to view the tapes.

Play Video 0:45 'Worst cover-up ever': Trump on apparent Khashoggi murder plot – video

The Saudi monarch, King Salman, in addition to May, spoke to Erdoğan on Wednesday, a day after the Turkish leader appealed to him to solve a festering crisis between the two regional power bases. An adviser to Erdoğan said the young prince had “blood on his hands” over Khashoggi. Turkish officials have suggested that they will release more information incriminating the Saudi state if King Salman does not move against the son he anointed as his heir 17 months ago.

Known for convictions that critics describe as brazen arrogance, Bin Salman struck several conciliatory tones during his appearance, the first towards archfoe Qatar, which Saudi Arabia has boycotted for more than a year. “Even though we have our differences with Qatar, in five years’ time they will be in a very different place,” he said. The comments were the first cracks in an unwavering hostility towards the small Gulf state.

More visibly, the Lebanese prime minister, Sadd Hariri – who was summoned to Riyadh by Bin Salman 11 months ago, then briefly detained – was sitting alongside him on the stage. It marked a remarkable reversal of fortunes for a man who had been discarded as a Saudi ally and forced to sign over local assets.

“Saad Hariri will be here for two more days,” Bin Salman said. “Don’t let anyone say he’s been kidnapped.