Businesses around the world rushed Saturday to contain a ransomware attack that has paralyzed their computer networks, a situation complicated in the U.S. by offices lightly staffed at the start of the Fourth of July holiday weekend.

It’s not yet known how many organizations have been hit by demands that they pay a ransom in order to get their systems working again. But some cybersecurity researchers predict the attack targeting customers of software supplier Kaseya could be one of the broadest ransomware attacks on record.

It follows a scourge of headline-grabbing attacks over recent months that have been a source of diplomatic tension between U.S. President Joe Biden and Russian President Vladimir Putin over whether Russia has become a safe haven for cybercriminal gangs.

Biden said Saturday he didn’t yet know for certain who was responsible but suggested that the U.S. would respond if Russia was found to have anything to do with it.

“If it is either with the knowledge of and or a consequence of Russia then I told Putin we will respond,” Biden said. “We’re not certain. The initial thinking was it was not the Russian government.”

Cybersecurity experts said the REvil gang, a major Russian-speaking ransomware syndicate, appears to be behind the attack that targeted the software company Kaseya, using its network-management package as a conduit to spread the ransomware through cloud-service providers.

“The number of victims here is already over a thousand and will likely reach into the tens of thousands,” said cybersecurity expert Dmitri Alperovitch of the Silverado Policy Accelerator think tank. “No other ransomware campaign comes even close in terms of impact.”

The cybersecurity firm ESET said there are victims in least 17 countries, including the United Kingdom, South Africa, Canada, Argentina, Mexico, Kenya and Germany.