U2's lead singer Bono has had emergency spinal surgery after suffering an injury while preparing for tour dates.
The 50-year-old musician was treated at a specialist neurosurgery clinic in Munich and is expected to stay there for a number of days.
He will then return to his home in Ireland to recuperate further, a spokesman said.
The band's next tour date in the US on 3 June has been postponed, while their Glastonbury appearance may be in doubt.
Further information about the possible impact on other tour dates would be announced in due course, a statement on the band's website added.
The Irish rock band was due to perform a string of gigs across North America, returning to Europe in August.
They were due to begin rehearsals on Tuesday for the opening date in Salt Lake City, and are scheduled to headline the Glastonbury Festival on 25 June.
U2 manager Paul McGuinness said of the tour: "We will make plans to reinstate the dates as soon as possible."
He added it was "unfortunate that we're inconveniencing fans" and that it was causing disruption to the band's 400-strong crew.
He advised fans to continue monitoring the U2 website for further news.
The 360 Degree tour earned the band around $109m (£75.3m) in 2009, making it the world's most profitable of the year, according to figures from Billboard magazine.