Jamal Khashoggi, a former Saudi newspaper editor and adviser to retired Saudi intelligence chief Prince Turki al Faisal, has written columns in the Washington Post criticising Saudi Arabia's policies.
A missing Saudi journalist critical of Riyadh was still being held at the kingdom's consulate in Istanbul, Turkish presidential spokesman Ibrahim Kalin said on Wednesday.
"According to information we have, this individual (Jamal Khashoggi), who is a Saudi national is still at the consulate as of now," Kalin told reporters during a televised news conference.
He said the Turkish foreign ministry and police were closely monitoring the case, adding that Ankara was in touch with Saudi officials.
"I hope it will be resolved peacefully," he said.
But a Saudi official told Reuters news agency Khashoggi was not in the country's consulate in Istanbul.
"He is not in the consulate nor in Saudi custody," the official said.
Khashoggi, who writes opinion pieces for the Washington Post, has been missing since Tuesday.
Two people close to Khashoggi, who left the United States last year fearing retribution for his critical views, are concerned about his whereabouts, saying he failed to emerge from Saudi Arabia's consulate in Istanbul on Tuesday.
Khashoggi's Turkish fiancée as well as a close friend contacted by Reuters said he had not exited the diplomatic mission for more than 7-1/2 hours after entering to secure documentation of his divorce so that he could remarry.
'I can speak when so many cannot'
The fiancée, who asked not to be named, said she had waited outside the consulate from 13:00 local time (10:00 GMT) and called the police when he had not re-appeared.
"I don’t know what's happening. I don’t know if he’s inside or if they took him somewhere else," she said by telephone from outside the compound.
Turkish and Saudi authorities, including the Istanbul consulate and the Saudi Embassy in Washington, did not immediately respond to requests for comment.
When asked whether the US State Department has sought information from the Saudis and the Turks, an official said, "We have seen these reports and are seeking more information at this time."
Khashoggi, a former Saudi newspaper editor and adviser to retired Saudi intelligence chief Prince Turki al Faisal, has lived in self-exile in Washington, DC, for more than a year.
Since then, he has written columns in the Washington Post criticising Saudi Arabia's policies towards Qatar and Canada, the war in Yemen, and a crackdown on the media and activists.
"I have left my home, my family and my job, and I am raising my voice," he wrote in September 2017. "To do otherwise would betray those who languish in prison. I can speak when so many cannot."
Khashoggi was in US on O-visa
Mohamad Soltan, an Egyptian-American activist who sees Khashoggi regularly in Washington, told Reuters that Khashoggi was in the United States on an O-visa, a temporary residency visa awarded to foreigners "who possess extraordinary ability" in the sciences, arts, education, and other fields and are recognised internationally, and had applied for permanent residency status.
All public protests are banned in Saudi Arabia, as are political parties. Labour unions are illegal, the media are controlled and criticism of the royal family can lead to prison.
Dozens of activists, clerics and intellectuals have been arrested in the past year in a crackdown on potential opponents of the kingdom's absolute rulers. Among them was economist Essam al Zamil, a friend of Khashoggi's, who was charged this week with joining a terrorist organisation, meeting with foreign diplomats and inciting protests.
Scores of businessmen were detained last November in Riyadh's Ritz-Carlton hotel in a separate campaign against corruption, unnerving some foreign investors. Most of them were released after reaching financial settlements with the authorities.