Russian cyber attacks are increasingly likely – they’ll target the electric grid risking US or NATO response.

The war in Ukraine is renewing fears of a Russian cyberattack on the U.S. — and the danger that such a strike could spiral into a wider conflict between the two powers.

U.S. officials have worried for years about a potential Russian hack of critical systems such as the electric grid, which energy experts say is vulnerable to attacks that could cut off power for hours, days, weeks or even months. The United States would almost certainly respond to such an assault on Americans’ daily lives — and lawmakers and policymakers have said the options would need to include military reprisals or cyber counterattacks, not just more economic sanctions.

Washington has never made it clear what kind of hack would trigger an escalation, despite years of warnings from security agencies that Russia is probably laying the groundwork for a critical infrastructure attack that the U.S. would be ill-prepared to prevent.

“All these hypotheticals about what constitutes an attack… suddenly could move from hypothetical to real literally in the next few days,” Senate Intelligence Chair Mark Warner (D-Va.) said in an interview this week.

Russian President Vladimir Putin issued an ominous warning to nations like the U.S. in his Thursday morning speech announcing the invasion — saying that “whoever tries to hinder us” will face “consequences that you have never faced in your history.”

Later that day, President Joe Biden said from the White House that “if Russia pursues cyberattacks against our companies, our critical infrastructure, we’re prepared to respond” — though he did not say what that response would entail.

Biden told Putin during a summit last summer that the United States’ critical infrastructure should be “off-limits” to digital assault, handing him a list of 16 sectors that the Department of Homeland Security has identified as especially vital to the national interest. Those include energy, water, financial services, health care, defense and the food supply.

But Russian government hackers have repeatedly made forays into U.S. critical infrastructure including the energy sector since 2016, the FBI and DHS have said. A 2019 “Worldwide Threat Assessment” by the U.S. intelligence community found that Russia has the capability to disrupt electrical distribution centers in the United States for “at least a few hours,” adding: “Moscow is mapping our critical infrastructure with the long-term goal of being able to cause substantial damage.”

A major attack on the U.S. grid could be especially devastating, plunging parts of the country into darkness for days or longer depending on the method of attack. Texas offered a peek at this scenario last year, when power failures triggered by cold weather left millions of people without power. More than 200 deaths were linked to the storm, two-thirds of them due to hypothermia.

Russian hackers demonstrated their ability to make such threats real by attacking Ukraine’s grid in 2015 and 2016, leaving portions of the country in the dark for several hours each time.

Criminal ransomware gangs based in Russia made the lesson even plainer for the U.S. in two critical infrastructure breaches last year — an attack on the East Coast’s main fuel supply line that triggered price spikes and gasoline shortages, and another that shut down more than a fifth of the nation’s beef supply.

If a cyberattack on the grid were massive enough, the U.S. might consider it an act of war on the same level as a physical strike against power plants. That could justify a response in kind by the United States and its NATO allies.