Washington DC - President Donald Trump on Thursday threatened to take severe action against Iran if its authorities "start killing people" who are protesting in the country, where an economic crisis has led to mounting civil unrest.

"I have let them know that if they start killing people, which they tend to do during their riots – they have lots of riots – if they do it, we are going to hit them very hard," Trump said during an interview with conservative radio host Hugh Hewitt.

Local media and official statements have reported that at least 21 people, including security forces, have died since the unrest began.

The movement, which originated with a shutdown on the Tehran bazaar on December 28 after the rial plunged to record lows, has spread nationwide and is now being marked by larger-scale demonstrations.

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Videos on social media showed that protests were again taking place on Thursday.

A large crowd was seen gathering on the vast Ayatollah Kashani Boulevard in the northwest of capital Tehran, according to social media images verified by AFP, while other images showed a crowd demonstrating in the western city of Abadan.

Trump shared a similar ominous morning earlier this month, saying the US was "locked and loaded and ready to go" if protesters were killed.

His latest comments come after US federal agents shot and killed a woman near protest activity in Minnesota, with the president doubling down on his disputed claim that the victim had attempted to run over an ICE officer with her car.