Sweden's Coop supermarket chain was forced to close all 800 of its stores because it could not operate its cash registers following one of the largest ransomware attacks in history.

The major food retailers's shutdown comes after a sophisticated ransomware attack - where cybercriminals employ a type of malicious software to block people from accessing their own data - was launched against U.S. tech provider Kaseya on Friday.

The ransomware gang, dubbed as REvil, is believed to have hijacked Kaseya's desktop management tool VSA, spreading a malicious update that infects tech management providers serving thousands of business.

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Huntress Labs, one of the first to alert authorities to the wave of infections, said Saturday that thousands of small companies could have been affected.

Kaseya, which is based in Miami, said it was working with the FBI to solve the issue, adding that only about 40 of its customers were impacted directly. It did not say how many of those were providers, which could in turn spread the malicious software to others.

The FBI announced on Saturday that it was working in coordination with the U.S. Cybersecurity and Infrastructure Security Agency.

"We encourage all who might be affected to employ the recommended mitigations and for users to follow Kaseya's guidance to shut down VSA servers immediately," the agency said.

The businesses that were hit had their files encrypted and were sent electronic messages demanding ransom payments. In some cases, firms were asked to pay millions of dollars.

"What we are seeing now in terms of victims is likely just the tip of the iceberg," said Adam Meyers, senior vice president of security company CrowdStrike.

Coop, one of Sweden's biggest supermarket chains, said a tool it used to remotely update checkout tills was hit in the attack, meaning payments could not be accepted.

"We have been troubleshooting and restoring all night, but have communicated that we will need to keep the stores closed today," Coop spokesperson Therese Knapp told Swedish Television.

Swedish news agency TT said Kaseya's technology was employed by the Swedish company Visma Esscom, which looks after servers and devices for several Swedish businesses.

A pharmacy chain as well as Sweden's state railway services also suffered disruption due to the attack.

"They have been hit in various degrees," Visma Esscom chief executive Fabian Mogren told TT.

Defence Minister Peter Hultqvist said the attack was "very dangerous" and was testament to the fact state agencies and businesses had to upgrade their protections against cyberattacks. 

"In a different geopolitical situation, it may be government actors who attack us in this way in order to shut down society and create chaos," he said.