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.