The United States government is warning state governors that foreign hackers are already carrying out cyberattacks against critical infrastructure systems throughout the country.

In a letter released Tuesday, National Security Advisor Jake Sullivan and Environmental Protection Agency Administrator Michael Regan warned that "disabling cyberattacks are striking water and wastewater systems throughout the United States."

The letter mentioned Iranian and Chinese cyber agents, specifically. Sullivan and Regan cited a case in which hackers accused of acting in concert with Iran's Revolutionary Guard had disabled a controller at a water facility in Pennsylvania.

Also mentioned was a Chinese hacking outfit called "Volt Typhoon" which is said to have "compromised information technology of multiple critical infrastructure systems, including drinking water, in the United States and its territories."

"These attacks have the potential to disrupt the critical lifeline of clean and safe drinking water, as well as impose significant costs on affected communities," the letter said.

Both Iran and China denied carrying out cyberattacks in the past.

The digital safety of water and sewage plants, as well as other municipal infrastructure connected to digital properties, have long been an area of concern for cybersecurity professionals that are historically lightly defended if defended at all.

A cyberattack intrusion at a Aliquippa, Pennsylvania booster facility, which monitors and regulates water pressure, drew national attention because the ailing controller was replaced with a message that read "YOU HAVE BEEN HACKED."

No damage to the water system was reported, but in a statement released at the time, an industry group called the Water Information Sharing and Analysis Center said "This may not be an isolated incident."

Tuesday's letter called on governors to "ensure that all water systems in your state comprehensively assess their current cybersecurity practices" and prepare for potential cyber incidents.