A host of clubs are being linked with the Korean, who has emerged as one of the SPL's best players since his move to Scotland in 2009.
Russian outfit Rubin Kazan are already understood to have had a bid rejected by Celtic and now offers from Germany look likely.
"The very much respected Bundesliga sides Werder Bremen and Bayer Leverkusen are holding an interest in Ki Sung-Yueng. Bremen already scouted him live," a spokesman for Ki's agents Global Sports Agent C2 told the Korea Times.
The player himself has previously said he is more than happy with life in Glasgow and he added to the Korea Times that he would only move for first-team action.
"The most important thing is to play first-team football. If I am getting the guarantee I would play regularly, then I would not care about the location of the club," he said.
Ki has two-years left on his current deal at Parkhead.