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. He 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 six people are reported to have been killed in Iran on Thursday 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.
Khamenei adviser Ali Larijani warned shortly afterwards that any US interference would destabilise the entire region and also harm American interests.
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 Tehran's nuclear weapon prospects - a claim disputed by Iran.
Iran retaliated with a missile attack on a major US military base in Qatar.
In Iran, six people were reported killed during a fifth day of protests on Thursday.
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.
Three people were killed in Azna and another in Kouhdasht, all in the west of the country. Fars did not specify whether those killed were demonstrators or members of the security forces.
Hengaw said the two killed in Lordegan were protesters, naming them as Ahmad Jalil and Sajjad Valamanesh.
BBC has not been able to independently verify the deaths.
Footage posted on social media showed cars being set on fire during battles between protesters and security forces.
BBC Persian has verified videos showing Thursday's protests in Lordegan, Tehran and Marvdasht in the southern Fars province.
Iranian officials earlier said a member of the country's security forces had been killed on Wednesday in the western city of Kouhdasht.
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.
By Tuesday, university students became involved and protests spread to several cities, with demonstrators chanting against the country's clerical rulers.
Many protesters have called for the end of Khamenei's rule, with some demanding a return to the monarchy.
The protests have been the most widespread since an uprising in 2022 that was sparked by the death of Mahsa Amini, a young woman accused of not wearing her veil properly.
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 effort to create instability would be met with a decisive response.
