According to Portuguese newspaper A Bola, the Colombian international admitted he could leave Porto after the World Cup this summer, putting the likes of Chelsea and Arsenal on alert.
Speaking to Colombian radio Antena 2, Martinez said: "I've not renewed with the club. It's something that we have been discussing during the season.
"As for my departure, I have a contract with FC Porto. After the World Cup, many things can happen but I am focused on continuing to do an excellent job for Porto and the rest will come with time."
Martinez went on to pay tribute to his coach, Luis Castro, but the fact he has not signed a new contract should worry officials and fans at Porto.
His €40m release clause would soften the blow of any loss, and the Portuguese giants have an excellent record when it comes to replacing their strikers, having recovered from the departure of Radamel Falcao.
Both Arsenal and Chelsea are expected to be in the market for a big-name forward at the end of the season, with Chelsea boss Jose Mourinho publicly critical of the effort put forward by his current crop of first-team strikers.
Arsenal too are overly reliant on Olivier Giroud and Arsene Wenger may look to strengthen this summer.