There was a £13million release clause in Martins' contract when he signed for the Toon, though he has recently been in discussions to remove the figure from his agreement.
Martins is a popular player with fans on Tyneside.
Even so, he has become frustrated at being asked to play out wide at times rather than down the middle.
He believes his best position is as a central striker.
A source close to Martins told The Sun: "There is a lot of interest in Oba and he is not sure where he stands at Newcastle."