(CNN) British rock band Radiohead have announced the release of hours of unheard material from the mid-1990s in response to ransom demands from unnamed hackers.

Last week cyber attackers stole archive material held by lead singer Thom Yorke and threatened to release it unless the band paid $150,000, according to a Facebook post from Radiohead. The recordings date from "around the time of OK Computer" -- Radiohead's third album released in 1997 -- the band said.

Radiohead lead singer Thom Yorke pictured in 2016.

"So instead of complaining -- much -- or ignoring it, we're releasing all 18 hours on Bandcamp in aid of Extinction Rebellion," reads the post, referring to the UK-based climate protest organization which says it uses "non-violent civil disobedience to achieve radical change."

Radiohead released an image of the stolen minidisc recorded at the time of the OK Computer sessions.

The material, which was stored on minidisk, will be available to download for $23 (£18) for the next 18 days, according to the post.

"Never intended for public consumption (though some clips did reach the cassette in the OK Computer reissue) it's only tangentially interesting," reads the post, which the band's PR firm says was written by lead guitarist and keyboardist Jonny Greenwood.

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