World Cup 2014: Cristiano Ronaldo limps out of training

Cristiano Ronaldo limped out of Portugal training on Wednesday amid concerns that his World Cup participation could be in doubt.

The Real Madrid forward, 29, left the field early with an ice pack strapped to his left knee.

The world player of the year has suffered from knee and thigh injuries since the end of the domestic season.

His country, who lost their opening Group D match 4-0 to Germany, face USA on Sunday.

And back-up goalkeeper Beto believes Ronaldo will be ready to face Jurgen Klinsmann's side at the Arena Amazonia in Manaus.

"Cristiano is 100% fit to play," he said. "Every match, if he starts to play, it is because he is fit to play, he's ready to play."

Ronaldo was not able to complete Wednesday's training session ahead of Portugal's game with the United States on Sunday

After declaring himself fit to lead his national side, Ronaldo played the full 90 minutes of Porgual's 4-0 defeat by Germany on Monday

Real Madrid forward Ronaldo missed two of Portugal's pre-tournament friendlies with knee and thigh problems