U.S. women's national soccer team co-captains Alex Morgan and Megan Rapinoe said they were shocked after their team's claims for equal pay were dismissed by a court.

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The duo, who helped the U.S. to a record fourth World Cup title last year, said they planned to appeal the decision, after a California judge on Friday threw out the players' claims that they were underpaid in comparison with the men's team.

"This decision was out of left field for us," said forward Morgan, appearing on a remote live feed on the "Good Morning America" program on Monday.

"We are fighters and we will continue to fight together for this."

The World Cup-winning team's long-running feud with U.S. Soccer has been a very public and bitter battle, and the players had been seeking $66 million in damages under the Equal Pay Act.

"The WNT [Women's National Team] has been paid more on both a cumulative and an average per-game basis than the MNT [Men's National Team] over the class period," the court said in its summary judgment.

Last year's Ballon d'Or winner Rapinoe said she questioned the logic of the summary judgment.

"If I earn one dollar every time I play and a man earns three dollars, just because I win 10 games and he only wins three games -- and so I make 10 dollars and he made nine dollars -- I'm not sure how that's me making more money," she said.

The court allowed complaints of unfair medical, travel and training to proceed to trial, which is scheduled to begin June 16.

Some of the players' high-profile backers, including women's sports pioneer Billie Jean King and presumptive U.S. Democratic presidential nominee Joe Biden, leapt to their defence after news of their legal setback broke.

"Don't give up this fight. This is not over yet," Biden wrote in a tweet to the team on Saturday, then telling U.S. Soccer: "Equal pay, now. Or else when I'm president, you can go elsewhere for World Cup funding."

The U.S. Men's National Team Players Association reiterated its support for the female players in a statement Monday.

"The USMNT players continue to stand with the WNT players in their efforts to secure equal pay," the group said.

"For a year and a half the USMNT players have made proposals to the Federation that would achieve equal pay for the USMNT and USWNT players."