Washington DC - President Donald Trump said on Friday that the US is "locked and loaded" to respond if Iran kills protesters, prompting Tehran to respond with its own warning.
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.
Trump said on his Truth Social platform that "if Iran shots (sic) 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 Trump that "US interference in this internal matter would mean destabilizing the entire region and destroying America's interest."
The US president "should be mindful of their soldiers' safety," Larijani wrote on X.
Ali Shamkhani, an adviser to supreme leader Ayatollah Ali Khamenei, said any US intervention would "be exposed to a response" calling Iran's security a "red line."
Trump in June took the unprecedented step of joining Israel's unprovoked war on Iran, ordering strikes on sites tied to its nuclear program.
Iran's leaders acknowledge legitimate frustrations
Iranian leaders including Larijani and President Masoud Pezeshkian have in recent days described peaceful protests over Iran's dire economy as legitimate and understandable.
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.
Iran's economy has been battered by years of crushing international sanctions over its nuclear program, with raging inflation and a collapsing currency.
The recent protests have affected 15 cities, largely in the west of the country, according to reporting in Iranian media.
The demonstrations are smaller than the last major incident in 2022, triggered by the death in custody of Mahsa Amini, who was arrested for allegedly violating Iran's strict dress code for women.
Her death sparked a nationwide wave of anger that left several hundred people dead including dozens of members of the security forces.