Turkish investigators have claimed video and audio recordings exist that prove Jamal Khashoggi was killed, a sign that Ankara is willing to keep up the pressure on Riyadh to back up its claims it has nothing to do with the dissident journalist’s disappearance.

US government officials told the Washington Post late on Thursday that their Turkish counterparts claimed the recordings from 2 October proved Khashoggi was murdered and his body dismembered during a visit to the Saudi consulate in Istanbul to pick up marriage paperwork.

The alleged audio evidence – which Turkish sources have also suggested exists in comments to the Guardian – is particularly strong, according to officials.

“You can hear his voice and the voices of men speaking Arabic,” a source told the Washington Post. “You can hear how he was interrogated, tortured and then murdered.”

While US officials said the contents of the supposed tapes had been described to them, it was not clear whether they had seen or listened to the recordings themselves.

Admitting the existence of any planted listening or video devices is a difficult balancing act for the Turks, who are wary of revealing any efforts or methods used to spy on foreign operations inside the country, and are also at pains to maintain a delicate political and trade relationship with Riyadh.

Saudi Arabia has vehemently denied all allegations it was involved in the journalist’s disappearance and says he left the consulate safely shortly after entering it last week.

Late on Thursday, however, it was announced that US investigators would assist a joint Turkish-Saudi investigation into how the 59-year-old vanished.

Riyadh previously said Turkish investigators could search the consulate, which as a diplomatic building is considered sovereign Saudi territory.