Bono ''doesn't know'' if U2 will tour again.

The 'One' hitmakers are currently on the road on their 'Experience and Innocent' concert series - which started in May and ends in November - and the 58-year-old singer admitted performing live night after night isn't as easy as it used to be, especially in the wake of his much-publicised mystery brush with death two years ago.

He said: ''I can't do as much as I used to. On previous tours I could meet a hundred lawmakers in between shows and now I know I can't do that.

''This tour is particularly demanding.

''Whether you have a face-off with your own mortality or somebody close to you does, you are going to get to a point in your life where you ask questions about where you're going.''

Asked if that means there won't be another tour, he told the Sunday Times magazine: ''I don't know. I don't take anything for granted.''

And drummer Larry Mullen - who requires an hour of physio an hour before and an hour after each concert to cope with the physical demands - suggested there is at least a ''really long'' break on the cards for the band, though doesn't think they'll quit music for good.

He said: ''We'll finish this and then there will be time to decide what we want to do next. I'd like to take a really long holiday...

''I assume there'll be another album. I don't know that anybody needs another U2 record or tour anytime soon.

''People could do with taking a break from us and vice versa.''

Guitarist The Edge is worried about his friend and frontman's health.

He said: ''These days, we try to save his voice. He has a good range. His top note would be a B these days, but he has hit C, which is what a top tenor would hit, and is very, very high. An opera singer would hit that maybe once a night...

''He has a very ambivalent attitude to his physical self. He doesn't naturally take responsibility for his physical wellbeing. Which is fine in your twenties, but you get to a certain point ... It is a difficult shift for him. If you spend too much time thinking you are old and past it, you probably can't do it any more.''