The Spain international faces an uncertain time at club level following the arrival of new coach Jose Mourinho at Real.
Casillas, who is currently on duty at the World Cup, told the Daily Star Sunday: "I am aware of the interest of both Manchester United and City.
"I'm always flattered whenever another club shows interest in my qualities as a goalkeeper. I have been at Real Madrid all my career and I still enjoy going to training every day.
"I enjoy meeting the fans too - but we all know how uncertain things are in football.
"I'm not going to say never. Right now Madrid would have to kick me out of the club before I consider a move.
"Who knows what will happen, though, if a big money offer comes in."