The 27-year-old Portuguese striker's contract with Real runs out in June 2015 but Spanish media have reported that he is unhappy at the club and may leave at the end of the season.
The player has been linked to a move to Paris Saint-Germain or back to Manchester United.
"I want to see out my contract at Real Madrid: I'm very clear about that. After that, well, I don't know what will happen in the future," he said in an interview published on Fifa.com.
Ronaldo, who joined Real Madrid from Manchester United in 2009, said he still believes it is possible for the club to win La Liga despite falling 18 points behind leaders and archrivals Barcelona.
"We haven't started the championship very well for sure. We know that La Liga's an uphill struggle for us now, but nothing is impossible in football," he said.
Real Madrid are in third place in the Spanish league, four points behind second-placed Atletico Madrid and have dropped even further behind Barcelona after they were held to a 0-0 draw at relegation-threatened Osasuna Osasuna on Saturday.
They looked a shadow of the team that romped to the Liga title last season.
Ronaldo also defended Real's Portuguese coach Jose Mourinho, who was jeered last month for benching goalkeeper Iker Casillas, a world and European champion with Spain, in what was seen as an attempt to assert his authority.
Many Real Madrid fans are unhappy at Mourinho's combative style and frequent insinuations of referee bias against his side and there are rumours of a rift in the dressing room with heavyweights like Casillas and Sergio Ramos complaining about his aloof style.