U.S. Soccer sued the union representing the world champion United States women’s national team in federal court Wednesday, a sudden escalation of a simmering labor fight over the team’s collective bargaining agreement.

In the lawsuit, U.S. Soccer, the national governing body for the sport, is seeking to have a court rule that the terms of the agreement — which expired in 2012 but has continued to be the guiding document over the relationship between the federation and star players like Hope Solo and Alex Morgan — remain valid. U.S. Soccer seeks no penalties; instead, it asked for “declaratory relief” stating that the players’ union must abide by a slightly modified version of the agreement that is set to expire in December.

U.S. Soccer said in the court filing that it “reluctantly” brought the action against the union representing the women’s team after the executive director of the union, Richard Nichols, threatened to repudiate the agreement and its no-strike clause in a negotiating session in New York.

Nichols, reached late Wednesday, rejected the accusation that he had raised the possibility of a labor action, saying, “There were no threats about strikes or work stoppages.” He said the players had merely “reserved our legal rights.”