Trump says Clinton's Syria policy would lead to third world war

US Republican presidential nominee Donald Trump said on Tuesday that Democrat Hillary Clinton's plan for Syria would "lead to world war three", because of the potential for conflict with military forces from nuclear-armed Russia.

In a Reuters interview focused largely on foreign policy, Trump said defeating the Islamic State (IS) group is a higher priority than persuading Syrian President Bashar al-Assad to step down, playing down a long-held goal of US policy.

Trump questioned how Clinton would negotiate with Russian President Vladimir Putin after demonising him.

On Syria's civil war, Trump said Clinton could drag the United States into a world war with a more aggressive posture toward resolving the conflict.

Clinton has called for the establishment of a no-fly zone and "safe zones" on the ground to protect non-combatants. Some analysts fear that protecting those zones could bring the United States into direct conflict with Russian warplanes.

"What we should do is focus on ISIS. We should not be focusing on Syria," Trump said, using another acronym for the militant group. "You're going to end up in World War Three over Syria if we listen to Hillary Clinton."

"You're not fighting Syria anymore, 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.

Trump said getting Assad to leave power was less important than defeating IS.

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

On Russia, Trump again knocked Clinton's handling of US-Russian relations while she was secretary of state and said her harsh criticism of Putin raised questions about, "how she is going to go back and negotiate with this man who she has made to be so evil," if she wins the presidency.

On Sunday, Trump reiterated his criticism of the Obama administration and its Iraqi allies for announcing the battle to recapture Mosul, Iraq's second largest city, from IS.

He described the attack as a "total disaster".

"We gave them months of notice. US is looking so dumb," Trump wrote in a tweet.

Clinton responded strongly to her opponent's Mosul statement, calling his comments "dangerous".

She said he had no plan to defeat IS.

"He’s basically declaring defeat before the battle has even started," Clinton said at a campaign event in New Hampshire. "He’s proving to the world what it means to have an unqualified commander in chief."

The two candidates are set to face off on 8 November. Polls show Clinton to be leading in most battleground states that Trump needs to win to clinch the presidency. A candidate needs to garner 270 Electoral College votes, awarded based on the results of individual states, to win the election.