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 as a "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 Thursday that the U.S. move was "profoundly reckless" and called on leaders to "halt the escalation spiral."

The Los Angeles Police Department tweeted that it is "monitoring the events developing in Iran," but it said there is no credible threat in LA.

"This Department is committed to ensuring the safety of our vibrant and diverse community, and we ask every Angeleno to say something if you see something," the LAPD said.

The Boston Police Department said it would "increase patrols," and Chicago will increase security in airports. Both cities said they are not aware of any credible threats but urged residents and visitors to report anything suspicious.

"As an added precautionary measure, we will have an increased security presence at our airports and have reached out to our public and private sector partners with reminders to stay vigilant after the holiday season," the Chicago Office of Emergency Management and Communications said in a statement.

Some in the Iranian American community expressed concern that police departments may target and stigmatize them.

Ryan Costello, policy director for the National Iranian American Council, told USA TODAY that as tensions heightened between the United States and Iran, Iranian Americans have seen a "corresponding threat to civil liberties."

"Iran is perceived as a very bad actor, as a terrorist and so forth" Costello said. "And therefore, Iranian Americans are subject to increased suspicion, screening at airports, xenophobia."

Costello said cities' concerns about being targeted by Iranian retaliation can be seen as "alarmist."

"I think it reflects how poorly people in general understand the Iranian American community and Iran. Iran, it's a negative actor in the region and so forth. But a lot of their actions are confined to the region," Costello said.

"I'm worried that people are going to say, 'Oh, OK, because you're Iranian American, you're under suspicion,' " Costello said.

Contributing: Nicholas Wu, Deirdre Shesgreen, Tom Vanden Brook and Kim Hjelmgaard