WASHINGTON -- A devastating cyberattack on U.S. government agencies has also hit targets worldwide with the list of victims still growing, according to researchers, heightening fears over computer security and espionage.

Microsoft said late Thursday that it had notified more than 40 customers hit by the malware which security experts say came from hackers linked to the Russian government and which could allow attackers unfettered network access.

"While roughly 80 percent of these customers are located in the United States, this work so far has also identified victims in seven additional countries," Microsoft president Brad Smith said in a blog post.

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Smith said the victims were found in Canada, Mexico, Belgium, Spain, Britain, Israel and the United Arab Emirates.

"It’s certain that the number and location of victims will keep growing," Smith said, echoing concerns voiced this week by US officials on the grave threat from the attack.

"This is not 'espionage as usual,' even in the digital age," Smith said.

"Instead, it represents an act of recklessness that created a serious technological vulnerability for the United States and the world."

John Dickson of the security firm Denim Group said many private sector firms which could be vulnerable are scrambling to shore up security, even to the point of considering rebuilding their servers and other equipment.

"Everyone is in damage assessment now because it's so big," Dickson said.

"It's a severe body blow to confidence both in government and critical infrastructure."

The threat comes from a long-running attack which is believed to have injected malware into computer networks using enterprise management network software made by the Texas-based IT company SolarWinds, with the hallmarks of a nation-state attack.

NSA WARNING

The National Security Agency, the secretive U.S. military intelligence operation, called for increased vigilance to prevent unauthorized access to key military and civilian systems.

"The recent SolarWinds Orion code compromise is one serious example of how on-premises systems can be compromised, leading to abuse of federated authentication and malicious cloud access," the NSA said.

Analysts have said the attacks pose threats to national security by infiltrating key government systems, while also creating risks for key infrastructure controls for systems such as electric power grids and other utilities.

The US Cybersecurity and Infrastructure Security Agency (CISA) said U.S. government agencies, critical infrastructure entities, and private sector organizations had been targeted by what it called an "advanced persistent threat actor."

CISA did not identify who was behind the malware attack but private security companies believe it was hackers linked to the Russian government.

U.S. Secretary of State Mike Pompeo also suggested involvement by Moscow on Monday, saying the Russian and Chinese governments have made repeated attempts to breach U.S. government networks.

President-elect Joe Biden expressed "great concern" over the computer breach.

Romney likened the cyberattack to a situation in which "Russian bombers have been repeatedly flying undetected over our entire country."

Senator Marco Rubio told Fox News: "It is massive. It is still, I would argue, probably persistent. It is still ongoing... It's a grave risk to federal, to state, to local governments, to critical infrastructure, to the private sector."

CISA said the computer intrusions began in March 2020 and the actor behind them had "demonstrated patience, operational security and complex tradecraft."

"This threat poses a grave risk," CISA said in a statement Thursday, adding that it "expects that removing this threat actor from compromised environments will be highly complex and challenging for organizations."

Hackers reportedly installed malware on software used by the U.S. Treasury Department and the Commerce Department, allowing them to view internal email traffic.

Media reports said the Department of Energy, which manages the government's nuclear arsenal, had also been breached.

SolarWinds said up to 18,000 customers, including government agencies and Fortune 500 companies, had downloaded compromised software updates, allowing hackers to spy on email exchanges.

After the attack was detected, CISA ordered federal agencies to power down the breached software.