Iranian President Hassan Rouhani said Saturday the United States committed a "grave mistake" in killing Iranian commander Qasem Soleimani and US President Donald Trump responded on Twitter, writing that if Tehran attacks American assets, the US will strike "very hard and very fast."The US has a list of 52 Iranian targets, Trump tweeted. The number was chosen to match the number of hostages taken in the 1979 takeover of the US Embassy.