In a joint statement, Britain's foreign minister Jeremy Hunt, his French counterpart Jean-Yves Le Drian and Germany's Heiko Maas said whoever was responsible for the Saudi journalist's disappearance must be held to account.
Khashoggi, a Washington Post contributor, vanished after entering the Saudi consulate in Istanbul on October 2 to obtain official documents for his upcoming marriage.
“Defending freedom of expression and a free press and ensuring the protection of journalists are key priorities for Germany, the United Kingdom and France,” the ministers said in a statement issued by the Foreign Office in London.
“In this spirit, light must be shed on the disappearance of Saudi journalist Jamal Khashoggi,” it said, indicating all three countries were treating the matter “with the utmost seriousness”.
“There needs to be a credible investigation to establish the truth about what happened, and — if relevant — to identify those bearing responsibility for the disappearance of Jamal Khashoggi, and ensure that they are held to account.
While expressing support for joint Saudi-Turkish efforts to look into his disappearance, they said the Saudi government must provide “a complete and detailed response”, indicating that such a message had been conveyed “directly” to Riyadh.
Turkish officials say they believe Khashoggi was killed inside the Saudi mission and lurid claims suggesting he was tortured and even dismembered have been leaked to the media.
Saudi Arabia warned Sunday it would retaliate against any sanctions imposed on the oil-rich kingdom over Khashoggi's disappearance, as the Riyadh stock market plunged on growing investor jitters.
From tech tycoons to media giants, a host of Western companies are now distancing themselves from the Gulf state, imperilling Crown Prince Mohammed bin Salman's much-hyped economic reform drive.