"We have selected 52 Iranian targets, some at a very high level and important to Iran and Iranian culture," Trump writes on Twitter.
If Iran attacks US or US assets, the US will strike "very quickly and very hard" against these 52 targets, the president writes.
The number 52 hints at the 1979-81 hostage crisis, when supporters of the Iranian Revolution held 52 US citizens hostage at the US Embassy in Tehran for 444 days.