US President Donald Trump, in his first comments on the case of disappeared Saudi journalist Jamal Khashoggi, said he was “concerned” and hoped the case would “sort itself out.”

“I am concerned about it. I don’t like hearing about it,” Trump told reporters on Monday. But he tempered his comments, saying: “Hopefully that will sort itself out. Right now nobody knows anything about it.”

“We call on the government of Saudi Arabia to support a thorough investigation of Mr Khashoggi’s disappearance and to be transparent about the results of that investigation,” Secretary of State Mike Pompeo said in a statement.

Khashoggi, a columnist for the Washington Post and critic of Saudi Crown Prince Mohammed bin Salman, went missing one week ago after entering the Saudi consulate in Istanbul. Turkish police have told multiple outlets they believe he was murdered on the premises.

In recent days, Americans lawmakers from across the political spectrum have warned the affair could harm US-Saudi relations.

“Just spoke to Senators Bob Corker and Ben Cardin about our shared concerns regarding the whereabouts and treatment of Mr Jamal Khashoggi,” Republican Senator Lindsey Graham tweeted Monday.

“We agree if there was any truth to the allegations of wrongdoing by the Saudi government it would be devastating to the US-Saudi relationship and there will be a heavy price to be paid – economically and otherwise.”

Democratic Senator Chris Murphy, who has previously called for the US to end support for Saudi Arabia over its war in Yemen, also expressed outrage over Khashoggi’s disappearance and possible murder.

“If this is true – that the Saudis lured a US resident into their consulate and murdered him – it should represent a fundamental break in our relationship with Saudi Arabia,” he tweeted Sunday.

Over the past year Khashoggi used his platform as a Washington Post columnist to criticize the policies of Saudi Arabia’s powerful young crown prince, from the war in Yemen to the arrest of women’s rights advocates.

But the disappearance of the journalist, however prominent, may not affect President Trump’s bet on the Saudi heir to the throne.

Saudi Arabia has long been a pillar of US policy in the Middle East, and while the relationship was rocky during President Obama’s tenure, Trump has embraced Riyadh.

In March, the State Department approved a $670 million arms sale to Saudi Arabia, part of a promised $110 billion in deals touted by Trump after his first presidential trip to the Saudi capital last year.

The kingdom has also assumed a critical role in ensuring global oil production remains steady as US sanctions against Iran’s petroleum sector come into effect on November 5.

“The request that America made to Saudi Arabia and other OPEC countries is to be sure that if there is any loss of supply from Iran, that we will supply that. And that happened,” the crown prince told Bloomberg on Friday in a wide-ranging interview.

“When President Trump became president, we’ve changed our armament strategy again for the next 10 years to put more than 60% with the United States of America,” he added.

The interview briefly touched on the Khashoggi case, with the crown prince stating he had no knowledge of the journalist’s fate or whereabouts and that he had left the Istanbul consulate.