Rio de Janeiro (AFP) - Rock band Aerosmith announced on Tuesday they were cutting short their tour of South America after frontman Steven Tyler flew back to the United States due to "unexpected medical issues".

"To everybody in South America - Brazil, Chile, Argentina and Mexico - I'm very sorry and feel like I've let you down," the 69-year-old wrote in a statement posted on Twitter, which did not specify the nature of the illness.

"I won't be able to continue the last four shows of this tour. I flew back to the US on doctor's orders last night."

"Please not to worry," he continued, "I am not in a life-threatening condition, but I need to deal with this right away and get some rest and medical care immediately in order to sustain and maintain my future performances... As they say, 'We humans make plans, and God laughs.'"

The band played on Monday night in Sao Paulo after performing on Thursday in Rio de Janeiro. The cities cut from the tour were Curitiba, Brazil; Santiago, Chile; Rosario, Argentina; and Monterrey, Mexico.

Aerosmith announced last year they were doing a "farewell" tour for 2017 but stopped short of saying the shows would be their finale.