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"We believe American troops should stay for at least the mid-term, if not the long-term," said Prince Mohammed bin Salman, who also holds the position of Saudi Defence Minister.

Bin Salman voiced concern that in the absence of the US military, Saudi's arch-nemesis Iran would be able to expand its unbridled influence over the region. He added that staying in Syria - where US forces are currently fighting remaining Islamic State fighters - would allow Washington to have a say in Syria's future.

"If you take those troops out from east Syria, you will lose that checkpoint," bin Salman said in an interview mid-way through his multi-city tour of the US.

"And this corridor could create a lot of things in the region," he added, referring to the so-called "Shia crescent" that spans from Lebanon, where Iran sponsors Shia group Hizballah, through to Syria where it backs the Assad regime and loyal Shia militias, to northern Iraq and Iran.

Bin Salman voiced concern that US withdrawal could free up an unrestricted supply chain through these areas, run by a combination of proxy militias and allied regimes.

Bin Salman added that Assad will likely stay in power, but voiced hopes that he would not become a "puppet" for Tehran, which was subsequently ridiculed by observers.

"Bashar is staying," he said. "But I believe that Bashar's interest is not to let the Iranians do whatever they want to do."

The Saudi leader's remarks come seemingly as a response to President Trump's address to Ohio workers on Thursday that US forces would pull out of Syria "very soon", lamenting what he said was Washington's waste of $7 trillion in Middle East wars.

The United States has more than 2,000 military personnel in eastern Syria, working with local militia groups to defeat the extremist Islamic State group while trying to keep out of Syria's broader civil war.

Trump's eagerness to quit the conflict flies in the face of a new US Syria strategy announced in January by then secretary of state Rex Tillerson - who has since been sacked.

Tillerson argued that US forces must remain engaged in Syria to prevent IS and al-Qaeda from returning and to deny Iran a chance "to further strengthen its position in Syria."

In a speech at Stanford University, he also warned that "a total withdrawal of American personnel at this time would restore Assad and continue his brutal treatment against his own people."

But Tillerson has gone after being dismissed in a tweet and Trump, who increasingly makes foreign policy announcements without seeking the advice of US generals or diplomats, wants out.

"We spent $7 trillion in the Middle East. And you know what we have for it? Nothing," Trump declared, promising to focus future US spending on building jobs and infrastructure at home.