He has also pushed for an agreement over Iran's nuclear programme.

Supreme leader Ayatollah Ali Khamenei on Sunday likened the recent protests to a "coup" and warned that a US attack would trigger a broad conflict.

"The Americans should know that if they start a war, this time it will be a regional war," he said.

Asked about the Iranian leader's warning, Trump told reporters on Sunday: "Of course he is going to say that.

"Hopefully we'll make a deal. If we don't make a deal, then we'll find out whether or not he was right," he said.

Oil's drop was helped by a stronger dollar, which came on the back of news that Trump had tapped Kevin Warsh to take the helm at the US central bank.

The president said Warsh, a former Morgan Stanley investment banker and Fed governor, "will go down as one of the GREAT Fed Chairmen, maybe the best".

Traders regard Warsh as the toughest inflation fighter among the final candidates, raising expectations of monetary policy that would underpin the greenback.

The choice also eased recent concerns about the Fed's independence following a series of attacks on incumbent Jerome Powell over his reticence to cut rates as quickly as the president wanted.