The U.S. is on heightened alert after Iran vowed ‘harsh retaliation’ for an American airstrike in Iraq and that killed a top Iranian general – as it is feared that sleeper cells may be ready to attack in major cities.

The killing of Gen. Qassem Soleimani, the head of Iran’s elite Quds Force, on Friday marks a major escalation in the standoff between the U.S. and Iran.

Major cities, including New York and Los Angeles, have already said they are stepping up security in the aftermath of the airstrikes to prevent any revenge attacks.

It comes amid fears that the Iran-backed Hezbollah militant group, which has been designated by the U.S. government as a foreign terrorist organization, may have sleeper cells in the U.S. that are ready to retaliate.

Former CIA operations officer, Sam Faddis, warned that Hezbollah sleeper cells were hiding in the U.S. and potentially had targets in its sights.

‘It is more than possible that Hezbollah has sleeper cells in the United States ready to strike,’ Faddis told DailyMail.com.

‘Based on what we know we must assume they do and that they can hit targets on U.S. soil at anytime without warning.’

Iran’s Supreme Leader Ayatollah Ali Khamenei has already warned that ‘harsh retaliation is waiting’ for America following the airstrikes.

It prompted authorities in New York City to take extra security measures with Mayor Bill de Blasio saying they would be protecting key locations from ‘any attempt by Iran or its terrorist allies to retaliate against the U.S.

De Blasio told a press conference Friday: ‘We have to assume this action puts us in a de facto state of war.

‘No one has to be reminded that New York City is the number one terror target in the United States. We have to recognize that this creates a whole series of dangerous possibilities for our city.

‘We have plenty of knowledge of previous efforts by Iranian proxies to scout and target locations in New York City. Hezbollah and others have made it a point in recent years… We know there’s been an effort to target and prepare to attack in New York City in the past.’

The mayor said the threat has changed significantly given the resources that a state such as Iran has compared to non-state organizations like Al Qaeda or ISIS.

‘It’s just a world of difference,’ de Blasio said. (Mail)