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 Qasem 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!"
Relations between Washington and Tehran have been tense since the 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 the Islamic republic.
The Iranian navy last week said it drove off American aircraft that flew close to an area where military exercises were underway near the Strait of Hormuz.
The military said three US aircraft were detected by Iran's air force radars after they entered the country's air defence identification zone.