U2 performing live with Bono (L) and Adam Clayton (R)
U2 won't release a new album until next year.
Although the band have performed live - appearing at The Oscars, The Golden Globes and on 'The Tonight Show Starring Jimmy Fallon' - and debuted two new tracks, 'Ordinary Love' and 'Invisible', they are still trying to perfect songs for their new album.
A source told Billboard magazine: "It seems to be taking longer for them to finish an album as they get older, but the great thing about U2 is that the whole of a record is always better than the sum of its parts.
"That magic that the band always seems to capture ... they have yet to capture it."
The album is being helmed by executive producer Danger Mouse and U2 - Bono, The Edge, Adam Clayton, and Larry Mullen - have scheduled further sessions with producers Ryan Tedder and Paul Epworth to complete the as-yet-untitled record.
Bono, 53, has previously said he is conscious of the band's age and having something to say on their 13th studio album.
He explained: "We're on the verge of irrelevance. You have to make stuff relevant to you and where you're at, make an honest account of what you're going through. If that's relevant to other people, great. But we don't know."
The singer also spoke about the inspiration behind the album, saying it goes back to when the group first got together in 1976.
He added: "We went back to why we wanted to be in a band in the first place. We were listening to the Ramones and Kraftwerk - you can hear both on 'Invisible'. It opened up a whole valve for me writing, it was like a damburst of sorts."
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