(Reuters) - U.S. authorities and major cities saw no imminent threats on Friday from potential Iranian retaliation over a U.S. air strike that killed Tehran’s most prominent military commander, but New York’s mayor said his city was bracing for an attack like never before.

New York Mayor Bill de Blasio said the security threat to his frequently targeted city changed significantly overnight considering the resources of a modern, major nation such as Iran compared to those of non-state organizations like al Qaeda or ISIS.

“We have never confronted in recent decades the reality of a war with a government of a large country with an international terror network at its behest,” the mayor told a news conference.

“New Yorkers deserve to know that we have entered into a different reality,” he said.

De Blasio and senior police officials called the news conference as Iran promised vengeance in response to the U.S. attack that killed senior military leader Qassem Soleimani.

Chad Wolf, acting secretary of the U.S. Department of Homeland Security, said his agency was working with law enforcement officials and others across the country and was ready to respond to any threats, though none had yet been detected.

New York has been the target of repeated attacks on civilians, including the destruction of the World Trade Center on Sept. 11, 2001, and failed attempts such as a truck bomb that did not explode in Times Square or a plot to bomb the subway that police thwarted.

In response, the New York Police Department has built intelligence and counter-terrorism divisions, posting officers in 14 foreign locations.

New York police were on “heightened vigilance” and New Yorkers could expect to see more uniformed officers, some heavily armed with long guns, at sensitive areas, Police Commissioner Dermot Shea said.

Greater security is typically seen on the subway, other mass transit, and tourist attractions at times of heightened alert.

But the escalation of tensions with Iran meant “a world of difference” and “we have to assume this action puts us in a de facto state of war,” the mayor said.

Iranian proxies such as the Hezbollah militant group have previously scouted New York locations for an attack, he said.

Other U.S. cities were also taking precautions.

Chicago increased security at airports and issued warnings to “stay vigilant after the holiday season,” officials said.

Los Angeles police asked people to remain on alert and said they were monitoring events in Iran.