In the announcement, which said the anthem rules are "inconsistent with the collective bargaining agreement and infringes on player rights," the NFLPA confirmed the league has agreed to discuss the issue further before the case moves forward:
NFL owners approved guidelines that force players to either stand on the sideline or remain in the locker room during the anthem. A player who kneels on the sideline could see his team fined, while teams will also be permitted to issue their own punishments to players.
The issue has been a hot-button topic since former San Francisco 49ers quarterback Colin Kaepernick started protesting racial injustice during the anthem in August 2016.
"I am not going to stand up to show pride in a flag for a country that oppresses black people and people of color," he told NFL Media at the time. "To me, this is bigger than football and it would be selfish on my part to look the other way. There are bodies in the street and people getting paid leave and getting away with murder."
The players' decision to kneel has consistently drawn the ire of United States President Donald Trump, who said in September that owners should fire players who don't stand, per Sophie Tatum of CNN.
"For a week, (that owner would) be the most popular person in this country," Trump said. "Because that's a total disrespect of our heritage. That's a total disrespect for everything we stand for."
A CNN poll conducted after Trump's comments showed 49 percent of respondents called the anthem protests the "wrong thing" for the players to do, while 43 percent said it was the "right thing" to do.
Kaepernick filed a grievance against the NFL in October, alleging teams colluded not to sign him after he became an unrestricted free agent in March 2017. The case is ongoing.