The Grateful Dead offshoot Furthur canceled a tour date after singer-songwriter-guitarist Bob Weir was unable to finish a concert in suburban New York City.

The Grateful Dead founding member "is unable to perform for the next several weeks," Weir's website said, citing "unforeseen circumstances."

Several videos posted online show the 65-year-old musician having to sit down onstage Thursday night during a performance in Port Chester, N.Y., with Furthur, a band he formed in 2009 with fellow Grateful Dead member and bass guitarist Phil Lesh.

As he was playing through "Unbroken Chain," from the Grateful Dead's seventh studio album "From the Mars Hotel," he fell over and was helped back up by crew members.

He was given a chair to sit in, and the band left the stage after the song.

The members returned after a few minutes without Weir. Lesh told the crowd Weir was suffering from a strained shoulder and the set would go on without him.

"Bob looks forward to resuming his 'Weir Here' broadcasts from TRI Studios in mid- to late June and performing his solo acoustic shows June 26-30," his website said.

TRI Studios, of the Tamalpais Research Institute, is Weir's multimedia performance studio in San Rafael, Calif., designed for broadcasting live high-definition video and audio streams directly over the Internet.