Iran's retaliation for the United States' targeted killing of its top general is likely to include cyberattacks, security experts warned on Friday. Iran's state-backed hackers are already among the world's most aggressive and could inject malware that triggers major disruptions to the US public and private sector.

Potential targets include manufacturing facilities, oil and gas plants and transit systems. A top US cybersecurity official is warning businesses and government agencies to be extra vigilant.

In 2012 and 2013, in response to US sanctions, Iranian state-backed hackers carried out a series of disruptive denial-of-service attacks that knocked offline the websites of major US banks including Bank of America as well as the New York Stock Exchange and NASDAQ. Two years later, they wiped servers at the Sands Casino in Las Vegas, crippling hotel and gambling operations.

The destructive attacks on US targets ebbed when Tehran reached a nuclear deal with the Obama administration in 2015. The killing early Friday in Iraq of Quds Force commander Gen. Qasem Soleimani long after Trump scrapped the nuclear deal completely alters the equation.

Our concern is essentially that things are going to go back to the way they were before the agreement, said John Hultquist, director of intelligence analysis at the cybersecurity firm FireEye. There are opportunities for them to cause real disruption and destruction.

Iran has been doing a lot of probing of critical US industrial systems in recent years trying to gain access but has limited its destructive attacks to targets in the Middle East, experts say.

Its not known whether Iranian cyberagents have planted destructive payloads in US infrastructure that could now be triggered.

Its certainly possible, Hultquist said. But we havent actually seen it.

Robert M. Lee, chief executive of Dragos Inc., which specialises in industrial control system security, said Iranian hackers have been very aggressive in trying to gain access to utilities, factories, and oil and gas facilities. That doesnt mean theyve succeeded, however. In one case in 2013 where they did break into the control system of a US dam garnering significant media attention Lee said they probably didn't know the compromised target was a small flood control structure 20 miles north of New York City.

Iran has been increasing its cyber capabilities but is not in the same league as China or Russia. State-backed Russian hackers have proven most adept at sabotaging critical infrastructure, witnessed in attacks on Ukraines power grid and elections, experts agree.

And while the US power grid is among the most secure and resilient in the world, plenty of private companies and local governments havent made adequate investments in cybersecurity and are highly vulnerable, experts say.

My worst-case scenario is a municipality or a cooperative-type attack where power is lost to a city or a couple of neighborhoods, Lee said.

Consider the havoc an epidemic of ransomware attacks has caused US local governments, crippling services as vital as tax collection. While theres no evidence of coordinated Iranian involvement, imagine if the aggressor instead of scrambling data and demanding ransoms simply wiped hard drives clean, said Hultquist.

You could see many cities and hospitals targeted at once with ransomware that encrypts data to make it unusable, but there is no way to decrypt it by paying a ransom, said cybersecurity veteran Chris Wysopal, the chief technical officer of Veracode.

The only known cybersecurity survey of US local governments, county and municipal, found that the networks of 28 per cent were being attacked at least hourly and that nearly the same percentage said they didnt even know how frequently they were being attacked. Although the study was done in 2016, the authors at the University of Maryland-Baltimore County dont believe the situation has improved since.

The top cybersecurity official at the Department of Homeland Security, Christopher Krebs, urged companies and government agencies to refresh their knowledge of Iranian state-backed hackers' past exploits and methods after Soleimanis death was announced. Pay close attention to your critical systems, he tweeted.

In June, Krebs warned of a rise in malicious Iranian cyberactivity, particularly attacks using common methods like spear-phishing that could erase entire networks: What might start as an account compromise, where you think you might just lose data, can quickly become a situation where youve lost your whole network.

Wysopal said the Iranians are apt to have learned a lot from the 2017 NotPetya attack, which the U.S. and Britain have attributed to state-backed Russian hackers and which caused at least $10 billion in damage globally. The worst cyberattack to date, it exploited unpatched software after being delivered through an unwitting Ukrainian tax software provider and spread on networks without human intervention.

When then-Director of National Intelligence James Clapper blamed Iran for the Sands Casino attack, it was one of the first cases of American intelligence agencies identifying a specific country as hacking for political reasons: The casinos owner, Sheldon Adelson, is a big Israel backer. Clapper also noted the value of hacking for collecting intelligence. North Koreas hack of Sony Pictures in retaliation for a movie that mocked its leader followed.

The vast majority of the nearly 100 Iranian targets leaked online last year by a person or group known as Lab Dookhtegan a defector, perhaps were in the Middle East, said Charity Wright, a former National Security Agency analyst at the threat intelligence firm InSights. She said its highly likely Iran will focus its retaliation on US targets in the region as well as in Israel and the US.

Iran is widely believed to have been behind a devastating 2012 attack on Aramco, the Saudi oil company, that wiped the data from more than 30,000 computers. It was also a victim of the Stuxnet computer virus. First uncovered in 2010, it destroyed thousands of centrifuges involved in Iran's contested nuclear programme and is widely reported to have been a US-Israeli invention.