The 29-year-old has been told by Cottagers boss Lawrie Sanchez that he can leave the club and Diop is now considering his options.
The likes of Newcastle, Sunderland and Birmingham are keen - but Pompey have made their move and are in pole position to conclude a deal, according to Sky Sports.
"I told the new manager in the summer I wanted a new challenge and now he has said yes," said Diop.
"He said 'now this is my team so if you want to leave go ahead'.
"I have many offers and I am just waiting. I will stay in England in the Premier League. I have had a good pre-season and I'm ready. I'm just waiting.
"I don't know if it will be anyone like Arsenal or Manchester United but I have good teams in England interested and I must wait. I don't want to say who they are yet.
"But I do expect to have left Fulham by the time the transfer window closes.
"I want a fresh challenge. And now the clubs know how much money Fulham want. But I don't have any control over that. I am just focused on my job."