Negotiations between the National Football League and the NFL Referees Association have reached a difficult stage as both parties work toward a new collective bargaining agreement, according to sources familiar with the talks.

The current collective bargaining agreement between the league and its officials is set to expire on May 31, creating a deadline for reaching a new deal. Discussions between the two sides have been ongoing since the summer.

Sources indicate that the labor talks are not progressing smoothly, though specific details about the points of contention have not been disclosed publicly. The negotiations involve working conditions, compensation, and other terms affecting the league's officiating crews.

The NFL Referees Association represents the officials who work regular season and playoff games across the league. Previous labor disputes between the NFL and its referees have resulted in work stoppages, most notably during the 2012 season when replacement officials were used for several weeks.

Both the league and the referees association have not commented publicly on the current state of negotiations. The talks are expected to continue as the May 31 deadline approaches.