The forthcoming soundtrack to 'Kurt Cobain: Montage Of Heck', the new documentary about the late NIRVANA frontman, will include an unreleased 12-minute acoustic song by the rocker.

Director Brett Morgen revealed the news via Twitter, writing, "Listening to a mind blowing 12 minute acoustic Cobain unheard track that will be heard on the montage of heck soundtrack."

A release date for the soundtrack has yet to be announced.

The film - which debuted at the Sundance Film Festival in January will premiere on HBO on May 4 - is billed as a raw and visceral journey through the singer's life, providing no-holds-barred access to his archives, including never-before-seen home movies, recordings, artwork, photography, journals, demos, personal archives, family archives and songbooks. Read more here.