The 23-year-old has reportedly been targeted by Juventus, who are said to be lining up a €4.5m (£3.8m) swoop this summer.
However, an official approach has not yet materialised.
"Many clubs observed him during the Dutch national matches," Sky Sports News quotes Jansen as saying. "I know there were Italian sides, including Juventus, but there have been no official bids and no negotiations are in progress.
"The Italian league is always of great interest, but it obviously depends on the club.
"Feyenoord would like to keep him but in football today if the right offer comes in they could decide to sell."
Janmaat arrived at De Kuip on a free transfer from Heerenveen last summer.