The 25-year old Colombian striker, who is currently on loan to homeland club Millonarios from Seattle Sounders, is set to move to the Portuguese giants this summer.
Montero's agent, Helmut Wennin of Colombiagol SA, told Maisfutebol that negotiations were ongoing regarding the proposed deal.
"It is not yet closed. We are negotiating," he said.
Montero added: "I have a contract with Millonarios and still have a game to the championship. There would be a righteous person in relation to the fans if speaking now from other clubs.
"If I'm going to Sporting? I continue with the same uncertainty. I have read news about the interest of several clubs, including Sporting, but these things the player is always the last to know."