UPDATE [11:15 p.m. ET]: Talks were continuing deep into the night. Little progress had been made. The league’s offer of a form of free agency at age 28 with eight years of tenure in MLS, first reported by ESPNFC.com, was not flying with the players. (Those who turn pro in their teens would have to wait as many as 10-12 years.) Neither side has responded to requests for comment.

Major League Soccer careened toward the first work stoppage in its 20-season history Tuesday as the sides in labor negotiations appeared to make little progress toward brokering a new collective bargaining agreement.

Three days before the season is scheduled to begin, the league and players’ union remained locked in discussions with federal mediators in Washington. Tuesday marked the final day of a three-day session, although talks could continue into Wednesday, if necessary.

As of Tuesday afternoon, neither side had issued any public statements. However, sources familiar with the talks said significant obstacles stand in the way of a deal.

“It seems like [the owners] are giving up nothing of substance,” said one source, who requested anonymity because he is not permitted to speak on the matter. “They have shot down all of the players’ proposals [on free agency]. It’s shocking. The owners are almost wanting a work stoppage.”

The previous CBA expired Jan. 31 but teams have conducted preseason as planned. No deadline is in place to reach a pact, but without a pathway to a deal, the players seem inclined to strike before the season starts.

Numerous players from around the league have attended this week’s talks. Several MLS owners, who joined the negotiations for the first time last week in New York, are also involved.

The regular season opener is Friday in Carson, Calif., with the reigning champion Los Angeles Galaxy facing the Chicago Fire. Chicago players are scheduled to fly to Los Angeles on Wednesday afternoon, but those plans might be cancelled if negotiations remain unsettled. The same goes for D.C. United’s CONCACAF Champions League quarterfinal against Costa Rica’s Alajuelense on Wednesday night at RFK Stadium.

Five years ago, the last time the league and players negotiated a CBA, MLS headed off a work stoppage with five days to spare. However, this time, the players are seeking some form of free agency — a device that runs up against the league’s single-entity business structure. All players are under contract with the league, not individual clubs, although teams have autonomy in pursuing talent and setting offers.

Unlike other U.S. sports leagues, MLS players do not have the freedom to directly negotiate with other teams inside the league when their contracts expire. They did make strides five years ago, agreeing to a two-stage, re-entry draft each winter for players of a certain age and experience.

MLS players out of contract do have the ability to leave the league altogether and sign overseas. In many cases, though, their previous MLS club retains their rights should they seek to return to the league.

Last week, Real Salt Lake owner Dell Loy Hansen said “there are some really fair offers that have come from the league. The perennial issue is that ‘we want free agency,’ but that can’t exist where everyone’s employed by the same employer. … That’s one of those real waste-of-time conversations.”

Owners are also concerned free agency would lead to escalating salaries. Teams are, however, constrained by a salary cap.

One source said the league’s only free agency proposal involved players age 32 or older with 10 years of experience with the same club. Both sides have declined to discuss specifics of the negotiations.

Players have said repeatedly that they understand MLS’s economics — most teams do not turn a profit and a slow-growth plan has allowed the league to blossom since its 1996 launch — and they are not seeking considerable salary increases. They do, however, want the ability to move between clubs and a stronger foundation for players who have helped build the longest continuous top-tier league in U.S. soccer history.

Last season, for example, about half of Chicago’s roster earned $80,000 or less. The average MLS salary is around $200,000, but the median is closer to $100,000. A handful of elite players account for almost 50 percent of overall payroll.

Asked Monday about the league’s reluctance to grant free agency, United union rep Bobby Boswell said:  “They say philosophically that is not what the owners bought into. They bought into a system of single entity and that they make the deals and control the deals, and that is what they want to continue to do.”

The threat of a work stoppage comes at a time of otherwise growing momentum for MLS and American soccer in general.

Attendance has grown and the league added expansion teams in Orlando and New York, raising the total to 20. Atlanta and a second L.A. team will start in 2017 and other markets for vying for expansion slots. MLS has signed high-profile international stars, most notably England’s Frank Lampard and Steven Gerrard and Spain’s David Villa, to lucrative contracts and U.S. national team players have continued migrating to MLS.