An average of national polls on the RealClearPolitics website since mid-October gives Clinton a lead of more than 5 percentage points, as Trump fights off accusations that he groped women and faces heavy criticism for suggesting he might not accept the result of the election if he loses.

Trump denies the accusations of sexual misconduct and says the election is rigged against him, although he has not cited widely accepted evidence to back that up.

On Tuesday, Trump said that Clinton's plan for fixing the Syrian civil war would "lead to World War Three," because of the potential for conflict with military forces from nuclear-armed Russia.

In an interview focused largely on foreign policy, Trump said defeating Islamic State was a higher priority than persuading Syrian President Bashar al-Assad to step down, playing down a long-held goal of US policy.

Clinton has called for the establishment of a no-fly zone and "safe zones" on the ground in Syria to protect noncombatants.

Her campaign responded in a statement later on Tuesday noting that both Republican and Democratic national security experts had deemed Trump ill-prepared to be commander in chief, and accusing Trump of "parroting" Russian President Vladimir Putin and playing on Americans' fears.

The two candidates have sparred in recent days over the US-backed Iraqi military push to take the northern Iraqi city of Mosul from Islamic State, which Trump described as a "total disaster."