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.