US President Donald Trump insisted on Tuesday that Mohammed bin Salman knew nothing about the 2018 killing of journalist Jamal Khashoggi by Saudi agents, offering a fierce defense of the visiting Saudi crown prince that contradicted a US intelligence assessment.

The controversy over the killing of Khashoggi, a Washington Post columnist and US-based critic of the Saudi leadership, flared again in the Oval Office in front of cameras as the kingdom's de facto ruler made his first White House visit in more than seven years.

US intelligence agencies concluded that bin Salman approved the capture or killing of Khashoggi at the Saudi consulate in Istanbul. The crown prince denied ordering the operation but acknowledged responsibility as the kingdom's de facto ruler.

"A lot of people didn't like that gentleman that you're talking about, whether you like him or didn't like him," Trump told reporters in the Oval Office with bin Salman sitting beside him.

"Things happened, but he knew nothing about it, and we can leave it at that."

Bin Salman said it had been "painful" to hear about Khashoggi's death but that his government "did all the right steps of investigation."

"We've improved our system to be sure that nothing happened like that. And it's painful and it's a huge mistake," he told reporters.

Trump, who chided the reporter who asked the Khashoggi question "to embarrass our guest," also praised the crown prince for doing an "incredible" job on human rights but did not elaborate.

Trump's treatment of bin Salman prompted a rebuke from Khashoggi's widow.

"There is no justification to murder my husband. While Jamal was a good transparent and brave man many people may not have agreed with his opinions and desire for freedom of the press," Hanan Elatr Khashoggi wrote on X, urging bin Salman to meet with her.

At the start of his visit, the crown prince was greeted with a lavish display of pomp and ceremony presided over by Trump on the South Lawn, complete with a military honour guard, a cannon salute and a flyover by US warplanes.

The warm welcome for bin Salman in Washington marks a high point for US-Saudi ties, which have suffered because of Khashoggi's murder.

Trump's predecessor Joe Biden traveled to the kingdom and met with the Saudi prince but he stopped short of hosting him at the White House.

Sitting next to Trump, bin Salman promised to increase his country's US investment to US$1 trillion from a US$600 billion pledge he made when Trump visited Saudi Arabia in May. But he offered no details or timetable.

A US$1 trillion investment in the US would be difficult for Saudi Arabia to pull together given its heavy spending on an already-ambitious series of massive projects at home, including futuristic megacities that have gone over budget and faced delays and stadiums for the 2034 World Cup.

Trump said he got a "positive response" about the prospects for Saudi Arabia normalising ties with Israel. But the crown prince made clear that while he wanted to join the Abraham Accords, he was sticking to his condition that Israel must provide a path to Palestinian statehood, which it has refused to do.

Trump told reporters that the two countries had reached a "defense agreement," without providing details, and that Saudi Arabia would buy advanced US-made F-35 fighter jets.

The sale of the stealth fighter jets to the kingdom, which has requested to buy 48 of the advanced aircraft, would mark the first US sale of the advanced fighter jets to Riyadh, a significant policy shift.

The deal could alter the military balance in the Middle East and test Washington's definition of maintaining what the US has termed Israel's "qualitative military edge."

Until now, Israel has been the only country in the Middle East to have the F-35. (Reuters)