Von Miller has launched an appeal against a four-game suspension for violating an unspecified NFL policy.

The NFL Players Association has confirmed that it is working with the Denver Broncos linebacker on his appeal, and that his case does not involve the steroids and related substances policy.

The 24-year-old wrote on Twitter: "I know I did nothing wrong. I'm sure this'll be resolved fairly. [Disappointed] Broncos have [to] open camp like this."

NFLPA spokesman George Atallah added: "The NFLPA is working closely with [Miller] to appeal his suspension. We are all very disappointed that confidentiality was breached."

Miller, who made 18.5 sacks, six forced fumbles and an interception in 2012, was recently voted the ninth best player in the NFL by his peers.