Billie Joe Armstrong Billie Joe Armstrong

Two days after breaking down onstage during the iHeartRadio music festival in Las Vegas, Green Day frontman Billie Joe Armstrong has checked into rehab, E! News reports.

"Green Day's Billie Joe Armstrong is seeking treatment for substance abuse," his rep told the site Sunday. "Green Day would like everyone to know that their set was not cut short by Clear Channel and to apologize to those they offended at the iHeartRadio Festival in Las Vegas. The band regretfully must postpone some of their upcoming promotional appearances."

VIDEO: Green Day's Billie Joe Armstrong flips out on stage

Armstrong, 40, caused a firestorm Friday night when he smashed his guitar, flipped off the camera and went on a profanity-filled rant at the festival. At the time, it was reported that the band's set had been cut by 20 minutes to give Usher more time.  "Give me a f---ing break. One minute left," Armstrong said. "I've been around since nineteen-eighty-f---ing-eight and you're going to give me one f---ing minute?" 

Armstrong was hospitalized in Italy earlier this month for an undisclosed illness. Green Day is set to drop their ninth studio album, Uno!, on Tuesday. Uno! Is the first of a three-part trilogy of albums with Dos! set to be released on Nov. 13.