Leaders in major American cities ramped up security measures in the wake of a U.S.-ordered airstrike that killed Iranian military leader Qassem Soleimani. New York City and Los Angeles officials announced they are monitoring for potential retaliation.

New York City Mayor Bill de Blasio said in statements late Thursday night and Friday that there are no known credible threats to the city but that the police department is increasing its presence. There will be more officers and more "unpredictability," de Blasio said.

The "finest police force in America is protecting" New Yorkers, de Blasio said at a news conference Friday. "Go on with your lives. Never be intimidated by the threat of terror, but be vigilant."

The Pentagon confirmed late Thursday that President Donald Trump ordered the killing of Soleimani asa "defensive action" as the Iranian general planned attacks against American diplomats and service members. Iran warned that a "harsh retaliation is waiting" for the United States.

The Department of Homeland Security said in a statement that there are no "specific, credible threats against our homeland" but that the DHS "stands ready to confront and combat any and all threats facing our homeland."

How we got here:Qassem Soleimani's killing is the latest in Iran-US tensions

Soleimani was the head of Iran's Quds Force and was involved in military and intelligence operations, including a two-day siege against the U.S. Embassy in Baghdad. His killing marked an escalation in the conflict between the United States and Iran and sparked fears about retaliation.

The National Iranian American Council's president, Jamal Abdi, said in a statement