President Donald Trump says the United States is "locked and loaded" to respond if Iran kills protesters, prompting Tehran to warn that intervention would destabilise the region.

Protesters and security forces clashed in several Iranian cities on Thursday, with six reported killed, the first deaths since the unrest escalated.

Shopkeepers in the capital Tehran went on strike Sunday over high prices and economic stagnation, actions that have since spread into a protest movement with political demands that has swept into other parts of the country.

Mr Trump said on his Truth Social platform that "if Iran shots and violently kills peaceful protesters, which is their custom, the United States of America will come to their rescue."

"We are locked and loaded and ready to go," he added.

That prompted the head of Iran's top security body, Ali Larijani, to warn Mr Trump that "US interference in this internal matter would mean destabilising the entire region and destroying America's interest".

The US president "should be mindful of their soldiers' safety", Mr Larijani added on X.

Ali Shamkhani, an adviser to Supreme Leader Ayatollah Ali Khamenei who previously was the council's secretary for years, separately warned that "any interventionist hand that gets too close to the security of Iran will be cut."

"The people of Iran properly know the experience of 'being rescued' by Americans: from Iraq and Afghanistan to Gaza," he added on X.

Iran's hard-liner parliament speaker Mohammad Bagher Qalibaf also threatened that all American bases and forces would be "legitimate targets."

Iranian leaders including Mr Larijani and President Masoud Pezeshkian have in recent days described peaceful protests over Iran's dire economy as legitimate and understandable.

Mr Pezeshkian said on Thursday that he and his government would "end up in hell", in the religious sense, if they failed to address economic hardship.

At the same time, officials have warned of a firm response to any instability.

On Friday, the prosecutor of the district of Lorestan, Ali Hasavand, was quoted on the Iranian judiciary's Mizan website as saying "any participation in illegal gatherings and any action aimed at disturbing public order, destroying property, disobeying law enforcement, inciting illegal gatherings… constitute crimes and will be treated with the greatest firmness by the courts".

"Certain opportunistic and hostile individuals are trying to undermine public security and peace by sowing chaos, disorder and committing murder.," he added.

"The justice system and the police will act with firmness and without any leniency against the rioters and those who undermine public security."

Battered economy