U2 are currently recording their 13th album - perhaps with a little help from Coldplay frontman Chris Martin.

Chris, who is a long time fan of U2, was seen arriving at Electric Lady Studios in New York City where Bono, The Edge, Adam Clayton and Larry Mullen Jr. have been working on their new album with producer Danger Mouse.

A source from fan site atu2.com seen the Coldplay singer’s arrival at the studio followed by Larry Mullen Jr. about two minutes later, writes the Irish Examiner.

"Shortly after that, a van arrived with three guitars - each with a white label that said 'U2/Edge' - and our reporter actually helped bring the gear into the studio.

"He saw Larry and Chris Martin together in another room before heading back outside."

Although Chris denied being in the studio when approached, U2 later posted a photo of themselves and Chris meeting the founder of the Inside Out Project, an art initiative which hosted a number of works throughout New York City in May.

In contribution to the project, U2 also did a video shoot of an acoustic version of their song ‘Sunday Bloody Sunday’ on the roof of Electric Lady Studios on Friday, the Irish Examiner reports.

One lucky Irish bar owner, whose residence is just next door to Electric Lady Studios managed to catch all the action and even snap a few pics.

“As the owner of Pour George sports bar located at 35 West 8th Street in Greenwich Village, I along with a few of my lucky patrons had the rare opportunity to see U2 perform live on a rooftop opposite my bar,” Garrity, a native of Knock, Co. Mayo, told us.

“They sang a slowed down version of ‘Sunday Bloody Sunday’ which was amazing.  Bono was super friendly to all of us and took the time out to chat. He said he will drop by next week for a pint at Pour George.”

Rolling Stone have said that the new album could be released in the fall: "We very much want to have a record out by the end of the year, September, October, November; that kind of time," bassist Adam Clayton said.

"We're working with Danger Mouse, who's a smart guy. He's on it; he's excited. It's a great team and feels very liberating at the moment – anything goes. We have an abundance of riches, we could make three or four different records and justify that to ourselves, but to make the best record you can, you have to steer away from the ones you can make easily. We're really trying to get into territory that we're not comfortable in. If that makes sense."