US President Donald Trump on Monday vowed that any attack by Iran would be met with a response "1,000 times greater in magnitude," after reports that Iran planned to avenge the killing of top general Qassem Soleimani.
A US media report, quoting unnamed officials, said that an alleged Iranian plot to assassinate the US ambassador to South Africa was planned before the presidential election in November.
"According to press reports, Iran may be planning an assassination, or other attack, against the United States in retaliation for the killing of terrorist leader Soleimani," Trump tweeted.
"Any attack by Iran, in any form, against the United States will be met with an attack on Iran that will be 1,000 times greater in magnitude!"
In response to the threat, Iran later warned the US against making a "strategic mistake".
"We hope that they do not make a new strategic mistake and certainly in the case of any strategic mistake, they will witness Iran's decisive response," Iranian government spokesman Ali Rabiei told a televised news conference.
Relations between Washington and Tehran have been tense since the 1979 Iranian revolution, and have spiraled since Trump unilaterally pulled out of a landmark international nuclear deal with Iran in May 2018.
In January, a US drone strike killed Soleimani in Baghdad, and Washington is pushing to extend an arms embargo on Iran that starts to progressively expire in October as well as reimposing UN sanctions on Tehran.