WASHINGTON - US President Donald Trump warned Iran Saturday that America has targeted 52 Iranian sites and Iran will be hit "very fast and very hard" if it attacks any American or US assets.

"We have targeted 52 Iranian sites (representing the 52 American hostages taken by Iran many years ago), some at a very high level and important to Iran and the Iranian culture," Trump tweeted on Saturday evening.

Trump's threat came after a US airstrike near Baghdad International Airport on Friday killed Iranian Major General Qassem Soleimani, commander of Iran's Islamic Revolution Guard Corps Quds Force, sparking outrage and vows to revenge from Tehran.

Earlier Saturday, the White House reportedly formally notified Congress of the US operation that killed Soleimani.

Major cities in America, including Washington, New York and Los Angeles, are on alert after the deadly incident while local authorities said there is no credible or specific threats toward the cities.

Meanwhile, anti-war protests took place across the United States on Saturday.

"For all who believe in peace, for all who are opposed to yet another catastrophic war, now is the time to take action," a coalition known as ANSWER, for Act Now to Stop War and End Racism, which organized the events, said in a statement.