Green Day 'begin ninth studio album'

Green Day singer Billie Joe Armstrong has revealed that the band have started work on material for their new album.

Although the group took nearly half a decade between 2004's American Idiot and last year's 21st Century Breakdown, the follow-up appears to already be under way.

Armstrong told Kerrang! that they recorded several demos during their European tour in October last year.

"We did some demos in Berlin, some in Stockholm, some just outside of Glasgow and some in Amsterdam," he said. "We wanted [to] get them down in some early form."

It is not currently clear whether Green Day aim to release the album this year or in 2011.