A pseudo-governmental electricity industry group in the U.S. has advised its members to boost their network security after reports emerged that a cyberattack downed a Ukrainian utility for six hours, Reuters reports.

The Dec. 23 incident left roughly 700,000 homes without power and is thought to be the first major blackout caused by hackers.

{mosads}The Electricity Information Sharing and Analysis Center (E-ISAC) called the blackout a “coordinated effort by a malicious actor” and last week urged its members to “do a better job” at layering digital security to keep out hackers.

The nine-page confidential document did not lay out specific weaknesses in the U.S. grid that might allow a similar breach to occur domestically, according to Reuters.

“There is no credible evidence that the incident could affect North American grid operations and no plans to modify existing regulations or guidance based on this incident,” a spokeswoman for E-ISAC said.

But experts have long warned that outdated systems and an increasingly connected grid have left vital infrastructure vulnerable.

Officials say hackers from Russia, Iran and China are all probing the U.S. power grid for weaknesses.

National Security Agency Director Adm. Michael Rogers has acknowledged to lawmakers that China and “one or two” other countries are capable of shutting down portions of critical U.S. infrastructure using a cyberattack.

The consequences if hackers moved from grid-mapping to a deliberate attack could be devastating. A blackout across 15 states and Washington, D.C., could cost the economy hundreds of billions of dollars, raise mortality rates at hospitals and cut the nation’s water supply, according to a recent study.

The Ukrainian blackout, which occurred at a single power company in the nation’s western regions, left large sections of the area it serves without power, including the regional capital Ivano-Frankivsk.

The E-ISAC report identified the systems integrator Galician Computer Co. as having worked for the downed utility, Prykarpattyaoblenergo, as well as two other utilities that were said to have been targeted but did not experience blackouts.

Galician told Reuters that it had provided software to only one of the three companies and was not directly involved in running the plant.

“According to reports from employees at that regional power firm, attacks were definitely carried out and led to blackouts,” a representative said. “We do not have any other information regarding this incident.”

The Ukrainian Security Service SBU was swift to blame Russia for the malware that caused the blackout. Relations between the two nations have been in a steep decline since Russia annexed Crimea last year and began supporting pro-Russian separatists in Ukraine.

“We found that the [malware] came from Russia,” SBU said. “It was an attempt to interfere in the system. But it was discovered and prevented.”