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 region.
"Trump should know that US interference in this internal matter would mean destabilising the entire region and destroying America's interests," he wrote.
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.
In retaliation, Iran launched a missile attack on a major US military base in Qatar.
In Iran, six people were reported to have been killed on a fifth day of protests on Thursday.
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