Donald Trump said on Tuesday that Hillary Clinton's plan for Syria would "lead to World War Three".

The Republican nominee told Reuters that defeating the Islamic State was more important than persuading Bashar al-Assad, the Syrian president, to step down.

"What we should do is focus on Isis. We should not be focusing on Syria," he said. "You're going to end up in World War Three over Syria if we listen to Hillary Clinton," he said.

"You're not fighting Syria any more, you're fighting Syria, Russia and Iran, all right? Russia is a nuclear country, but a country where the nukes work as opposed to other countries that talk," he said.

"Assad is secondary, to me, to ISIS."