Turkey's foreign ministry has summoned Saudi Arabia's ambassador for consultations over the disappearance of journalist Jamal Khashoggi after he entered the Saudi consulate in Istanbul, Turkish sources said on Thursday.

They said the Saudi envoy was called in on Wednesday to clarify the whereabouts of Khashoggi, a critic of Riyadh's foreign policy and its crackdown on dissent who left Saudi Arabia last year saying he feared retribution for his views.

"Yesterday the Saudi ambassador was summoned to the foreign ministry and the issue of Khashoggi was discussed," one of the sources said.

"Efforts are being made to clarify allegations about Khashoggi. We believe a positive outcome will emerge."

He said contacts were continuing to "overcome the problem".

The source gave no further details but Turkish broadcaster NTV said the ambassador told Turkish officials he had no information about Khashoggi.

Turkey's Deputy Foreign Minister Yavuz Selim Kiran told the Saudi envoy that the issue "should be cleared up immediately," NTV said.

Khashoggi's fiancée and a close friend said he vanished after entering the Saudi mission in Istanbul on Tuesday.

Saudi Arabia's Consulate General in Istanbul said in a statement on Wednesday that Khashoggi had left the consulate building shortly after his appointment on Tuesday.

It said the consulate was working with Turkish authorities "to uncover the circumstances" of his disappearance.

She said that he was "stressed and sad" that he was forced to go to the building.

Khashoggi was required to surrender his mobile phone, which is standard practice in some diplomatic missions. Hatice said he left the phone with her and told her to call an adviser to Turkish President Recep Tayyip Erdogan if he did not return.

Hatice said she waited for Khashoggi outside the consulate from about 1pm (10:00 GMT) until after midnight and did not see him leave. She returned when the consulate reopened on Wednesday morning.

No one at the Saudi embassy in Ankara was available to comment on Thursday and an official at the consulate, asked about the envoy's summons, referred back to the statement.

The Saudi Foreign Ministry said in a tweet that its Undersecretary for Political and Economic Affairs Adel Merdad held talks in Riyadh with Turkey's ambassador Erdogan Kok.

Erdogan's spokesman said on Wednesday Turkey believed Khashoggi was still inside the Saudi consulate in Istanbul, challenging the Saudi account.

A former editor of the al-Watan newspaper and a short-lived Saudi TV news channel, Khashoggi was for years seen as close to the Saudi royal family. He served as an adviser to senior Saudi officials.

After several of his friends were arrested, his column was cancelled by the Al-Hayat newspaper and he was allegedly warned to stop tweeting, Khashoggi left Saudi Arabia for the US, from where he wrote opinion pieces for the Washington Post and continued to appear on Arab and Western TV channels.

"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."

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