Saudi Arabia stood by denials that the kingdom killed a missing journalist but backed a "thorough" probe, the United States said on Tuesday, as Secretary of State Mike Pompeo paid a supportive trip to the Gulf state.

President Donald Trump dispatched the top US diplomat on an urgent mission to Riyadh to defuse a crisis over Jamal Khashoggi, an insider turned critic of the monarchy who was last seen on October 2 when he entered the Saudi consulate in Istanbul to sort out marriage paperwork.

Turkish officials say that Khashoggi, who lived in the United States and contributed to The Washington Post, was killed inside the consulate. Pompeo will speak to Turkish leaders in Ankara on Wednesday, the two countries said.

Pompeo met in Riyadh with the top leadership of Saudi Arabia including Crown Prince Mohammed bin Salman, who not long ago basked in international praise for his domestic reforms but whose reputation has soured over his mounting crackdown on dissent.

As Pompeo met the crown prince over dinner, Trump tweeted that he had telephoned and that the 33-year-old heir apparent "totally denied any knowledge of what took place in their Turkish consulate".

The crown prince "told me that he has already started, and will rapidly expand, a full and complete investigation into this matter. Answers will be forthcoming shortly", Trump wrote.

Trump had earlier threatened "severe punishment" if it is proven that Khashoggi was killed in the consulate. But he also mused aloud that "rogue" elements carried out the operation and has ruled out cutting sales of US weapons, of which Saudi Arabia is the largest foreign buyer.

Trump's remarks contradict reports by CNN and The Wall Street Journal alleging Saudi Arabia was weighing admitting the killing but characterising the death as an unintended mistake.

Alliance on display

State Department spokeswoman Heather Nauert said Pompeo was "candid and direct" with the Saudi leaders. But there was no palpable tension, with Pompeo full of smiles.

"We are strong and old allies. We face our challenges together," the crown prince told Pompeo in the palace decked out with chandeliers and gold-plated furniture, to which Pompeo said, "Absolutely."

Meeting King Salman, Pompeo "thanked the king for his commitment to supporting a thorough, transparent and timely investigation of Jamal Khashoggi's disappearance", Nauert said.

Saudi Arabia has been closely linked to the United States for more than a half century as a critical oil supplier, with relations warming under Trump as the US administration aggressively confronts the kingdom's regional rival Iran.

Just as Pompeo was visiting Riyadh, the US Treasury Department slapped fresh sanctions on Iran, punishing a paramilitary group and a network of businesses accused of financing it.

But the Saudis have faced intense criticism over the Khashoggi incident, which comes just after US senators nearly succeeded in cutting off support for Riyadh's air campaign against rebels in Yemen, which a UN report says has killed thousands of civilians.

There have also been sharp words targeting Mohammed bin Salman, known by his initials MBS, from US senators on both the Republican and Democratic sides, amid calls for Congress to halt US arms sales to the kingdom.

Saudi Arabia warned before Pompeo's visit that the leading oil exporter can retaliate, with an official quoted in state media warning that the kingdom "plays an effective and vital role in the world economy" and can "respond to any action with a bigger one".

Turks step up probe

Turkish authorities have stepped up their investigation into Khashoggi's disappearance, with police searching the Saudi consulate for eight hours into Tuesday morning, taking a sample of the garden soil.

On Tuesday, Turkish media reported that Saudi Arabia's consul to Istanbul, Mohammed al-Otaibi, had returned to Riyadh as Turkish authorities prepare to search his residence.

The UN human rights chief called on Tuesday for the lifting of the immunity of officials who might be involved in Khashoggi's disappearance.

Due to the seriousness of the case, the immunity generally accorded to diplomats "should be waived immediately", Michelle Bachelet said.

Britain, France and Germany also released a rare joint statement saying they were treating Khashoggi's disappearance "with the utmost seriousness" and calling for a "credible investigation".

'Davos in Desert' unravels

The controversy has not only troubled the Saudis' traditional Western allies, which are key arms suppliers to the kingdom, but undermined efforts by Mohammed bin Salman to present himself as a modernising ruler.

An investment conference seen as a platform for the crown prince and dubbed the "Davos in the Desert", scheduled to take place in Riyadh next week, has been hit by a string of prominent cancellations.

JPMorgan Chase CEO Jamie Dimon, Ford chairman Bill Ford and Larry Fink, the head of investment giant BlackRock, were among the latest business barons to cancel plans to attend.

US Treasury Secretary Steven Mnuchin said he still plans to attend but would take "into account" any new information.

The controversy also threw into doubt a $400 million deal the Saudi Public Investment Fund negotiated with Hollywood's most powerful talent agency as part of the crown prince's drive for a foothold in the entertainment industry.