Republican presidential candidate Donald Trump has said Hillary Clinton's foreign policy plan in Syria would trigger World War Three.
He said the US should focus on defeating the so-called Islamic State rather than convincing Syrian President Bashar al-Assad to stand down.
Mrs Clinton has been proposing a no-fly zone over Syria, which some say could lead to conflict with Russian jets.
Mr Trump also attacked Republicans for not uniting behind his candidacy.
"If we had party unity, we couldn't lose this election to Hillary Clinton," he told Reuters news agency at Trump National Doral golf resort in Miami, Florida.
The Republican standard-bearer struck an apocalyptic tone when criticising his Democratic rival's plan to control Syrian air space.
"You're going to end up in World War Three over Syria if we listen to Hillary Clinton," Mr Trump 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 also suggested Mrs Clinton would be unable to talk with Russian President Vladimir Putin after her sharp criticism of him.
Mr Trump questioned "how she is going to go back and negotiate with this man who she has made to be so evil" if she is elected president on 8 November.
His warning of confrontation with Russia echoes concerns raised last month at a congressional hearing by the highest-ranking military officer in the US armed forces.
Chairman of the Joint Chiefs Marine General Joseph Dunford told lawmakers a "no-fly zone" in Syria could spell war with Russia.
"Right now, senator, for us to control all of the airspace in Syria it would require us to go to war, against Syria and Russia," Gen Dunford told the Senate Arms Services Committee.