NEW YORK (AP) - Willie Nelson has come off his tour because of a "breathing problem."

The 86-year-old singer apologised on Twitter late on Wednesday (Aug 7), writing "I need to have my doctor check out."

Nelson had just finished performing with Alison Krauss in Toledo, Ohio, and he was next scheduled to appear on Friday in Grand Rapids, Michigan.

He wrote "I'll be back."

Farm Aid says Nelson will resume his tour on Sept 6.

Nelson will perform at the Farm Aid concert with John Mellencamp, Dave Matthews, Neil Young and Bonnie Raitt on Sept 21 in Wisconsin.

Nelson's tour is scheduled to end Nov 29 in Thackerville, Oklahoma.

Nelson cancelled his appearance at the Outlaw Music Festival due to illness last summer.

He also was forced to cancel several dates in 2018.