Britain, France and Germany insisted Sunday that "light must be shed" on the whereabouts of journalist Jamal Khashoggi as they called for a credible investigation into his disappearance.

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.

Following the statement, Hunt spoke to media at the foreign secretary's Chevening country residence, and challenged Riyadh to explain what happened to Khashoggi.

"The country that can help us get to the bottom of this is Saudi Arabia," he said.

"If, as they say, this terrible murder didn't happen then where is Jamal Khashoggi? That is what the world wants to know.

"If they have got nothing to hide then they will, and should, co-operate."

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.afp