U.S. Secretary of State Mike Pompeo has called on Saudi Arabia to support a thorough, transparent investigation into the disappearance of Saudi journalist Jamal Khashoggi, who has not been seen since entering the country's consulate in Istanbul last week.
Turkish officials have said Khashoggi was murdered inside the diplomatic outpost, while Saudi Arabia says he safely left the building.
WATCH: Turkey wants evidence
?"We have seen conflicting reports on the safety and whereabouts of prominent Saudi journalist and Washington Post contributor Jamal Khashoggi," Pompeo said in a statement late Monday. "State Department senior officials have spoken with the Kingdom of Saudi Arabia through diplomatic channels about this matter."
His comments came hours after U.S. President Donald Trump expressed concern about the situation.
"Right now nobody knows anything about it. I do not like it," Trump told reporters at the White House.
Turkey on Monday sought permission to search the Saudi consulate to look for clues. Saudi Crown Prince Mohammed bin Salman said last week that Riyadh was "ready to welcome the Turkish government to go and search our premises," because it had "nothing to hide" about the missing journalist.
But it was not immediately clear whether Turkish officials were granted access to the consulate after Monday's request. Saudi officials say the Turkish investigators' claim that Khashoggi was murdered are "baseless."
Turkish President Recep Tayyip Erdogan said Saudi officials need to prove that Khashoggi left the building after arriving last Tuesday to get a document for his upcoming marriage. His Turkish fiancee, Hatice Cengiz, who waited for him outside the consulate, said he never came out of the building.
"We have to get an outcome from this investigation as soon as possible. The consulate officials cannot save themselves by simply saying, 'He has left,'" Erdogan said on a visit to Budapest.
Khashoggi, who had been critical of the Salman government, has been living for a year in self-imposed exile in the United States after a Riyadh crackdown on dissent in the kingdom.
Protesters gathered outside the Saudi consulate Monday demanding to know what had happened to Khashoggi. Banners read, "We will not leave without Jamal Khashoggi."
U.S. Republican Senator Lindsey Graham said he and other lawmakers "agree if there was any truth to the allegations of wrongdoing by the Saudi government, it would be devastating to the U.S.-Saudi relationship and there will be a heavy price to be paid - economically and otherwise."
A Turkish official told the Reuters news agency, "The initial assessment of the Turkish police is that Mr. Khashoggi has been killed at the consulate of Saudi Arabia in Istanbul. We believe that the murder was premeditated and the body was subsequently moved out of the consulate."
Reuters' Turkish sources did not say how they thought Khashoggi was killed.
Police said earlier that about 15 Saudis arrived in Istanbul on two flights last Tuesday and were at the consulate at the same time as Khashoggi.
The Washington Post editorial board said Sunday that Saudi Arabia, Turkey and the United States "bear inescapable responsibility" to act in response to Khashoggi's disappearance.
The board said Saudi Arabia has to identify the 15 officials who were at the consulate and exactly what happened inside. Turkey, it said, must back up its conclusion Khashoggi was killed by making public any evidence it has.
Amnesty International’s Middle East research director, Lynn Maalouf, said if the reports of Khashoggi's killing are true, it "would be an abysmal new low" and "amount to an extrajudicial execution. This case sends a shockwave among Saudi Arabian human rights defenders and dissidents everywhere, eroding any notion of seeking safe haven abroad."