WASHINGTON — 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."

Ali Larijani, secretary of the Supreme National Security Council, condemned Trump’s remarks, saying he “should know that American interference in this internal issue is equivalent to chaos across the entire region and the destruction of American interests”.

“We consider the positions of the protesting merchants separate from those of the destructive elements,” Larijani added in a post on X.

“The people of the US should know that Trump began the adventurism. They should take care of their own soldiers.”

Larijani’s remarks likely referenced the US’s wide military footprint in the region. In June, Iran attacked Al Udeid airbase in Qatar after the US strikes on three Iranian nuclear sites during Israel’s 12-day war with Iran.

In his post, the US president did not specify what action Washington could take against the Iranian authorities.

Iranian officials earlier said a member of the country's securities forces had been killed on Wednesday in the western city of Kouhdasht.

Footage posted on social media showed cars set on fire during running battles between protesters and 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.

President Masoud Pezeshkian has said he will listen to the "legitimate demands" of the protesters.

The protests have been the most widespread since an uprising in 2022 sparked by the death in custody of Mahsa Amini, a young woman accused by morality police of not wearing her veil properly. — Agencies