U2 have returned from Fez in Morocco where the quartet has been writing new songs. Bono & Co. have decamped to the North African country with long-term production collaborators Brian Eno and Daniel Lanois, though the pair's involvement has been strictly about songwriting.

"It's the first time we've worked with Brian and Dan in a purely songwriting capacity," drummer Larry Mullen Jr. revealed. "So it's very different, quite experimental and kind of liberating because of that."

However, the musicians are reluctant to say whether these new tracks will be on their next album or not.