Washington (CNN) US authorities are increasing vigilance and fortifying defenses as Iran warns of a "harsh revenge" in the aftermath of a US drone strike that killed Qasem Soleimani, the powerful commander of the country's elite Quds Force.

The major escalation by the US, conducted in Baghdad as tensions in the region flared following the Iranian-backed siege of the US embassy in Iraq, has inevitably invited Iranian retaliation, experts say, and national security officials in the US are preparing for attacks on multiple fronts, including American outposts abroad, soft targets in the US and in cyberspace.

Law enforcement officials said there is no indication of an active or imminent plot within the US, but local police in cities across the country announced on Friday that they were strengthening patrols to defend against potential violence inspired by the killing or even the activation of possible underground terrorist networks seeded by Iran in the US and across the world.

Meanwhile, FBI counterterrorism investigators were reviewing possible threats from supporters of Hezbollah, the Iranian-backed militia whose broad reach is known to extend outside of the Middle East to Europe, South America and the US, according to current and former US law enforcement officials.

Investigators are looking through tips and investigative leads and redoubling attention to suspects already on the FBI's radar to determine whether they require deeper scrutiny, a US official said.

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