Donald Trump on Saturday drew what may come to be seen as the most significant red line of his presidency in a three-part tweet, vowing specific military action against Iran if it "strikes any Americans, or American assets."

"We have targeted 52 Iranian sites (representing the 52 American hostages taken by Iran many years ago), some at a very high level & important to Iran & the Iranian culture" and "if Iran strikes any Americans, or American assets... Iran itself, will be hit very fast and very hard," Trump wrote in a series of tweets.

Trump reiterated his warning in a series of tweets early Sunday, saying the United States just spent $2 trillion on military equipment.

The string of tweets come after the White House formally notified Congress on Saturday of the US operation that killed Iranian military commander Qasem Soleimani on Thursday.