Turkey yesterday said it was "concerned" by days-long protests that have engulfed neighbouring Iran, warning against any escalation in the unrest.

"Turkey is concerned by news the protests in Iran... are spreading, causing casualties and also the fact that some public buildings were damaged," the foreign ministry said in a statement, adding "common sense should prevail to prevent any escalation."

Iranian President Hassan Rouhani has tried to play down the unrest, which began over economic grievances in second city Mashhad last Thursday but quickly turned against the Islamic regime as a whole with chants of "Death to the dictator".

The five-day unrest, the biggest challenge to the Islamic regime since the 2009 mass demonstrations, has so far claimed 21 lives.

Turkey -- which was hit by protests against President Recep Tayyip Erdogan (then premier) in 2013 -- said it "attaches the utmost importance to the maintenance of peace and stability in friendly and brotherly Iran."