The 29-year-old former Sunderland and Tottenham player has been a regular for the League One team, but was left out of Saturday's victory at MK Dons.
He has made 97 appearances for Posh, and only signed a new three-year contract in June.
"I wouldn't want to keep someone here who is unhappy and not focused," boss Grant McCann told the club website.
"We are all disappointed... but the reasons behind it I can understand."
Peterborough goalkeeper Ben Alnwick has asked to be put on the transfer list for "personal reasons", the club says.