Iran vowed "harsh retaliation" for the death of Gen. Qassem Soleimani, the head of Iran's elite Quds Force, which marked a major escalation in the standoff between Washington and Iran, one that has careened from one crisis to another since President Donald Trump withdrew from the 2015 nuclear deal and imposed crippling sanctions.
"What we have to assume based on previous knowledge is that the Iranians would have an interest in prominent targets, well-known American locations," Mayor Bill de Blasio said. "So we'll obviously keep special watch over those. We also know that deterrence is achieved by being unpredictable, so if we think bag checks are necessary, we'll do bag checks, whatever it's going to take, (but) we're always going to respect people's rights and liberties."
Police in New York are deploying to "strategic locations" throughout the city, even though there are presently no credible threats, officials said.
"There are no specific, credible threats to New York City," Police Commissioner Dermot Shea said. However the NYPD fears a "ripple effect" from the strike that de Blasio said put the two countries in a "de facto state of war."
WATCH: Mayor, NYPD commissioner talk security after airstrike kills Iranian general
The NYPD said the city remains the country's top terror target, and the counterterrorism command had already developed a list of locations where a reprisal attack could occur.
"If they were ever to attempt directly or indirectly an attack on New York City, they would be bringing horrible, horrible results to their own people," de Blasio said. "It's really important for people to understand: Think of every war we've been in, we have never, ever dealt with an adversary that had this particular capacity...This is a full-blown government of a major modern nation."
Police also noted how Iranian proxies have previously been caught surveilling locations in the city for a potential attack, and those sites are also getting extra attention.
The NYPD has been on heightened alert for New Year's Eve in Times Square and for this Sunday's Anti-Defamation League march against anti-Semitism.
Patrick Warren, the Chief Safety Officer of the MTA, said New Yorkers can expect to see more officers with rifles, canine units and even officers in plain clothes who will ride the subways.
Similar steps were being taken at potential targets around the country, including in Los Angeles.
"While there is no credible threat to Los Angeles, the LAPD is monitoring the events developing in Iran," the LAPD tweeted. "We will continue to communicate with state, local, federal and international law enforcement partners regarding any significant intel that may develop."
The Secure Community Network, the nation's top organization dedicated to protecting synagogues and Jewish institutions around the country, has issued a notice to security professionals, synagogue leaders and law enforcement about the killing of Qassem Soleimani, urging them to be vigilant and report suspicious activity.
"The potential for threats to the Jewish communities outside of Israel cannot be disregarded," the notice issued Thursday night says. "Hezbollah and Iran have in the past responded to attacks via overseas terrorism, such as the 1994 suicide truck bombing of the AMIA Jewish community center in Buenos Aires, Argentina, and the 1992 attack at the Israeli Embassy in Buenos Aires."
The warning also notes that Soleimani's killing "comes after more than a year of escalating tensions between Iran and the United States."
The United States urged American citizens to leave Iraq "immediately" following the Friday airstrike at Baghdad's international airport that killed Soleimani and nine others, Iran's state TV reported Friday. The State Department said the embassy in Baghdad, which was attacked by Iran-backed militiamen and their supporters earlier this week, is closed and all consular services have been suspended.
Around 5,200 American troops are based in Iraq, where they mainly train Iraqi forces and help to combat Islamic State militants.
Iran's Supreme Leader Ayatollah Ali Khamenei warned that "harsh retaliation is waiting" for the U.S. after the airstrike, calling Soleimani the "international face of resistance." Khamenei declared three days of public mourning and appointed Maj. Gen. Esmail Ghaani, Soleimani's deputy, to replace him as head of the Quds Force, .
Iranian President Hassan Rouhani called the killing a "heinous crime" and vowed his country would "take revenge."
Thousands of worshippers in the Iranian capital Tehran took to streets after Friday Muslim prayers to condemn the killing, chanting "Death to deceitful America."
The targeted strike, and any retaliation by Iran, could ignite a conflict that engulfs the whole region, endangering U.S. troops in Iraq, Syria and beyond. Over the last two decades, Soleimani had assembled a network of heavily armed allies stretching all the way to southern Lebanon, on Israel's doorstep.
However, the attack may act as a deterrent for Iran and its allies to delay or restrain any potential response.
(The Associated Press contributed to this report)
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