It had been claimed that with his position in the team reportedly under threat, the 28-year-old had stayed behind after the main session to work on his shooting.
Rooney has now confirmed those reports, as well as being critical of the media.
"Sometimes wonder what the press are getting at. I said from the start I want to do everything I can to make sure I'm ready for these World Cup games and as part of that I was doing extra training a week before the squad joined up. That's exactly what I did yesterday, my own extra training because that's what I wanted to do," he wrote on Facebook.
Rooney's failure to find the net last time out against Italy extended his goalless run at the World Cup to nine matches.