US President Donald Trump said on Tuesday he would lift sanctions on Syria to offer it a chance for "greatness" after Bashar al-Assad's fall, as he cast himself as a peacemaker in the Middle East.

On a state visit to Saudi Arabia primarily aimed at securing billions of dollars of investment, the billionaire president took aim both at the US left and right, who he said had intervened in the region in the guise of "nation builders" but "wrecked far more nations than they built."

Trump renewed his offer for a better US relationship with longtime nemesis Iran, although he warned the nation's rulers of consequences if ongoing talks with his administration failed.

In his most significant announcement, Trump said he would end sweeping US sanctions on Syria, in place for decades and ramped up dramatically during the iron-fisted rule of Assad, who was overthrown by Islamist-led forces in December.

"I will be ordering the cessation of sanctions against Syria in order to give them a chance at greatness," Trump said in a speech in Riyadh.

"The sanctions were brutal and crippling," he said. "But now it's their time to shine."

Trump will at least "say hello" to Syria's interim president, Ahmed al-Sharaa, in Riyadh on Wednesday, a US official said.

Secretary of State Marco Rubio will hold more extensive talks in Turkey this week with Syria's foreign minister, Asaad al-Shaibani, Trump said.

Shaibani welcomed the lifting of sanctions, telling state news agency SANA it was a "pivotal turning point for the Syrian people."

In announcing the move, Trump said he was listening to the appeals of Saudi Arabia's de facto ruler, Crown Prince Mohammed bin Salman – who joined the crowd in cheering the announcement – as well as President Recep Tayyip Erdogan of Turkey, the longtime patron of the triumphant Syrian Islamists. (AFP)