Vladimir Putin repeated Russia's insistence that sanctions against Iran are "unreasonable" (File Photo)

Russian President Vladimir Putin on Thursday said any use of force by the US against Iran would lead to disaster as tensions escalate between Washington and Tehran.

"The US says it does not rule out the use of force... This would be a disaster for the region," Putin said during an annual televised phone-in with screened questions posed by Russian viewers.

"It would lead to a surge in violence and an increase in the number of refugees," he said, adding that the consequences of intervention would be "difficult to calculate".

Moscow has backed Tehran in its stand off with the United States since Washington pulled out of an international 2015 nuclear agreement with Iran last year.

Putin repeated Russia's insistence that sanctions against Iran are "unreasonable".

The comment comes at a time of growing antagonism between Iran and the US following two waves of still unexplained attacks on Gulf shipping, which Washington has blamed on Tehran.

Iran has denied any involvement and hinted that the US might have orchestrated them itself to provide a pretext for the use of force against the Islamic republic.

On Thursday tensions also heightened after the Pentagon confirmed that Iranian forces had shot down a US naval surveillance drone.

The US has insisted the aircraft was in international air space but Iran has said the drone had violated Iranian airspace.