The United States said Thursday that Iranian authorities may have killed more than 1,000 people in a crackdown on demonstrations, which Washington cast as the clerical regime's worst-ever internal challenge.

The damning account came as the US put new pressure on its arch-enemy by not ruling out sending more forces to the region. "It appears the regime could have murdered over 1,000 Iranian citizens since the protests began," Brian Hook, the US pointman on Iran, told reporters.

He acknowledged that information was difficult to verify in Iran, which has severely curtailed the internet, but said: "We know for certain it is many, many hundreds."

Hook said that "many thousands" of Iranians have been wounded and that at least 7,000 protesters have been detained.

President Donald Trump, receiving UN diplomats at the White House, called the crackdown "brutal" and a "horrible situation" as he vowed to respond "strongly" to any threat from Iran to US interests.