The 33-year old centre-back linked up with City for training on Tuesday, prompting suggestions that the club are interested in signing him.
The former Standard Liege, Newcastle United, AC Milan, FC Twente, Sporting CP and Malaga defender joined the Addicks in October last year after spending time at Sheffield Wednesday following a short stint with QPR.
Additionally, Onyewu has made 69 international appearances for the USA.
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