The hackers behind a mass extortion that has targeted hundreds of companies worldwide have demanded $70 million to end the attack.

They made the demand late Sunday (July 4) on a blog typically used by the REvil cybercrime gang.

It's a Russian-linked group that is among the world's most prolific extortionists.

The gang broke into U.S.-based IT firm Kaseya, and then targeted its clients.

Known victims include Swedish grocery chain Coop, which had to shut hundreds of stores after cash registers were knocked offline.

Experts say other targets include schools and public-sector bodies.

About a dozen countries have been hit.

On Sunday the White House said it was reaching out to victims to provide assistance.

An executive at Kaseya would only say that the firm was aware of the ransom demand.