De Gea's current deal expires in just over a year, with Real Madrid believed to be keen on acquiring his services.
"It's not true that the deal is done," Mendes was quoted as saying by AS. "I am negotiating his contract renewal but we haven't signed anything yet. As of this moment, we will see what happens.
"We're holding talks and at this moment, it seems he could continue with United, but as I say, we haven't signed anything yet."
The 24-year-old, who faces fresh competition for game-time with the arrival of ex-Barcelona keeper Victor Valdes, joined United from Atletico Madrid in an £18million move in 2011.