Striking Major League Soccer referees on Monday accused substitute officials of making a litany of mistakes in last weekend's fixtures as their bitter labor dispute with league chiefs rumbled on.

The Professional Soccer Referees Association (PSRA) -- the union that represents officials normally used in MLS -- said stand-in referees had made an array of blunders in recent games.

"Points were lost, red cards were incorrectly given while other red cards were incorrectly not given, and 'stonewall' penalty kick decisions were missed," the PSRA said in a statement on Twitter.

The PSRA statement was the latest salvo in the body's dispute with MLS and its Professional Referees Organization (PRO).

Major League Soccer kicked off its new season last month with the league forced to use substitute referees to officiate in games after contract talks between the PSRA and the MLS's PRO broke down.

MLS commissioner Don Garber has expressed frustration at the stalemate, accusing the PSRA of not negotiating "fairly" during talks.

In an interview with The Athletic last week, Garber said the league was prepared to continue using substitute referees for as long as needed, and insisted feedback from MLS players and coaches had been positive.

"We have officials that we think are doing a really good job, our players think they're doing a good job, our coaches think they're doing a good job, and they're going to continue to work and in a way that will be supported by PRO until there's a resolution," Garber told The Athletic.

However, Garber's remarks were swiftly rebutted by the players' union, the MLS Players Association (MLSPA), which said there was in fact widespread dissatisfaction at the standard of refereeing.

"It is grossly inaccurate to say that players think that the current group of referees are doing a 'good job,'" the MLSPA said in a response to Garber's remarks.

"In fact, players are very clear that the replacements are under-trained, lack experience and are not nearly at the level that a league of MLS's stature deserves."

- 'Freakish calls' -

Those criticisms were underscored by last weekend's round of fixtures in MLS, where multiple officiating controversies prompted criticism from players and coaches.

Charlotte FC coach Dean Smith said his team had been on the wrong end of several dubious decisions in a 2-1 loss to Nashville.

The most notable was the failure to award what looked like a clear penalty when Charlotte striker Patrick Agyemang was bundled over in the area.

"Maybe my interpretation is different but that's a stonewall penalty," Smith said after the game.

"I can't see any reason why it wasn't given, and I also don't know why the red card wasn't given for denial of a goal scoring opportunity. Some really, really freakish calls, which was disappointing."

Los Angeles Galaxy's experienced Japan international Maya Yoshida, meanwhile, appeared to wince when asked about refereeing following his team's 3-3 draw with St. Louis on Saturday.

"I'm going to get fined," Yoshida said. "I hope (PSRA) makes a deal with the league as soon as possible. That's a shame. That's it."

Meanwhile, Montreal coach Laurent Courtois was furious following his team's chaotic 4-3 defeat to Chicago Fire in the Windy City, which included three penalties, a red card and two goals in second half stoppage time.

Courtois said referee DeShun Beard had failed to send off Chicago's goalkeeper for a handball outside the area, missed an offside that led to a Chicago goal and that one goal arose from an incorrectly awarded corner.

"Something was taken away from my guys and it's hard to swallow under these conditions," Courtois said.

"They work too hard for this."