US President Donald Trump has warned Iran's authorities against killing peaceful protesters, saying Washington will come to their rescue.
In a brief post on social media, he wrote: We are locked and loaded and ready to go, but gave no further details.
A senior adviser to Iran's Supreme Leader Ayatollah Ali Khamenei responded by saying Trump should be careful if he intervened, warning of potential chaos across the Middle East.
At least eight people are reported to have been killed in Iran after almost a week of mass protests sparked by worsening economic conditions.
In Friday's post on Truth Social, Trump wrote: If Iran shots [sic] and violently kills peaceful protesters, which is their custom, the United States of America will come to their rescue.
In his post, the US president did not specify what action Washington could take against the Iranian authorities.
In June, the US carried out strikes against Iran's nuclear sites on Trump's orders. American officials later argued that the strikes had significantly set back the prospect of Tehran building a nuclear weapon - a claim disputed by Iran.
Shortly after Trump's latest social media post, Khamenei adviser Ali Larijani issued a warning of his own.
Trump should know that US interference in this internal matter would mean destabilising the entire region and destroying America's interests, he wrote.
But for some of the protesters, intervention from the US would be welcome.
They [security forces] are afraid and they shake to the bones when Mr Trump says something or Mr (Israeli Prime Minister Benjamin) Netanyahu says something, a young woman protesting in Tehran told the BBC's Newshour programme.
Preferring to stay anonymous for her own safety, she said protesters had been asking for US support for years, because the security forces believe that if Mr Trump says something, he will do [it], and they know if anything happens, they would have to take the consequences.
Eight people are reported to have been killed since protests started on Sunday.
Two people died in clashes between protesters and security forces in the south-western city of Lordegan, according to the semi-official Fars news agency and the human rights group Hengaw, which said they were protesters, naming them as Ahmad Jalil and Sajjad Valamanesh.
Three people were killed in Azna and another in Kouhdasht, all in the west of the country, Fars reports. It did not specify whether they were demonstrators or members of the security forces.
The protests began on Sunday in Tehran among shopkeepers angered by another sharp fall in the value of the Iranian currency, the rial, against the US dollar on the open market.
By Tuesday, university students were involved and protests had spread to several cities, with people chanting against the country's clerical rulers.
Many protesters have since been calling for the end of Khamenei's rule. Some have said they want a return to the monarchy.
President Masoud Pezeshkian has said he will listen to the legitimate demands of the protesters.
But the country's Prosecutor-General, Mohammad Movahedi-Azad, warned that any attempt to create instability would be met with a decisive response.
