Mr Trump later told reporters at the White House: "I am concerned about it". There are some pretty bad stories going around.
"Yes, this is of serious concern, the apparent enforced disappearance of Mr. Khashoggi from the Saudi Consulate in Istanbul", UN human rights spokeswoman Ravina Shamdasani told a Geneva news briefing.
The Saudi writer spent previous year in the U.S.in self-imposed exile, after he fled the kingdom amid a crackdown on intellectuals and activists who criticized the policies of Crown Prince Mohammed bin Salman.
However, Yasin Aktay, said to Reuters that he did not find claims by the Saudi officials about the lack of camera footage honest.
Self-exiled Saudi dissident Jamal Khashoggi said earlier this year that the Saudi government has been moving toward nationalist radicalism.
She said Mr Khashoggi was required to surrender his mobile phone, which is standard practice in some diplomatic missions.
After several of his friends were arrested, his column was cancelled by the al-Hayat newspaper and he was allegedly warned to stop tweeting, Mr Khashoggi left Saudi Arabia for the USA, from where he wrote opinion pieces for the Washington Post and continued to appear on Arab and Western TV channels.
The Saudis have called the allegations "baseless", insisting Khashoggi left the building through an exit that wasn't the main building.
Meanwhile, Turkish President Recep Tayyip Erdogan said "God willing, we will not be faced with the situation we do not desire" when asked by journalists about Khashoggi.
Trump's comments coincided with a vehement denial from Saudi Arabia's ambassador to the US, Prince Khalid bin Salman bin Abdulaziz, who said claims that Khashoggi had been killed or detained by Saudi authorities were "absolutely false, and baseless", according to a statement obtained by CNN.
That harshness corresponds with the growing concerns internationally about the direction Saudi Arabia is taking under Prince Mohammed and his father.
OSMAN ORSAL/REUTERS/NewscomNearly a week after the Saudi journalist Jamal Khashoggi disappeared, suspicions are rising that the regime in Riyadh has silenced him.
Turkey's foreign ministry said Tuesday that authorities will search the Saudi consulate in Istanbul as part of an investigation into the disappearance of journalist Jamal Khashoggi last week.
The Post said "a person close to the investigation" shared the image with them, without elaborating.
The Committee to Protect Journalists said it was alarmed by reports Khashoggi may have been killed inside the consulate. Turkish officials told Reuters over the weekend they believed he had been killed inside the consulate.
The former government adviser, who turns 60 on October 13, has lived in self-imposed exile in the United States since past year to avoid possible arrest.
But after several of his friends were arrested, his column was cancelled by the al-Hayat newspaper and he was allegedly warned to stop tweeting, Mr Khashoggi left Saudi Arabia for the US.
Motasem pointed out that the family has taken legal action regarding Jamal's disappearance in coordination with the Saudi government and the measures are proceeding within a legal framework.