US President Trump has warned the US is "targeting" 52 Iranian sites and will strike "very fast and very hard" if Tehran hits Americans or US assets.
His comments followed the US assassination of Qasem Soleimani, a top Iranian general, in a drone strike on Friday.
Iran has vowed to avenge his killing, as thousands across Iran and Iraq mourned the military commander's death.
Trump wrote on Twitter that Iran "is talking very boldly about targeting certain USA assets" in response to the general's death.
He said the US has identified 52 Iranian sites, some "at a very high level & important to Iran & the Iranian culture, and those targets, and Iran itself, WILL BE HIT VERY FAST AND VERY HARD" if Tehran strikes the US.
"The USA wants no more threats!" Trump added.
The president said the 52 targets represented the 52 Americans held hostage in Iran for more than a year from late 1979, after they were seized at the US embassy in Tehran.
Two previous US presidents, Barack Obama and George W Bush, had rejected strikes on the general as too risky.
Soleimani, 62, was widely seen as the second most powerful figure in Iran after Ayatollah Khamenei. The Quds Force he led reported directly to the ayatollah, and Soleimani was hailed as a heroic national figure.