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.
Iran has vowed to avenge his killing.
Mr 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 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.
Mr Trump’s tweets came hours after a huge funeral procession for Soleimani was held in Baghdad, where he was killed.
Several rocket attacks shook the area shortly after the procession, including one in the Green Zone near the US embassy. Several were fired north of the Iraqi capital at Balad air base, which houses US forces.
The Iraqi military said nobody had been hurt in the attacks.
No group has yet said it was behind the bloodshed. Pro-Iranian militants have been blamed for other recent attacks.
