NEW YORK -- The NFL Players Association sent an open letter to team owners calling for an end to the lockout of on-field officials and hinting that it's a violation of the contract between the players and the league.

The players were locked out for 4½ months last year, and they emphasized in the letter, posted Sunday on the union's website, that a lack of a safe working environment exists with replacement officials working the games.

Domonique Foxworth, president of the NFLPA, Drew Brees and Scott Fujita are among the union members who signed it.

"Your decision to lock out officials with more than 1,500 years of collective NFL experience has led to a deterioration of order, safety and integrity," the NFLPA wrote. "This affirmative decision has not only resulted in poor calls, missed calls and bad game management, but the combination of those deficiencies will only continue to jeopardize player health and safety and the integrity of the game that has taken decades to build."

The NFL did not immediately respond to a request for comment.

The NFL met with the officials' union Sunday, two people familiar with the talks told The Associated Press. It was uncertain whether progress was made in an attempt to reach a new collective bargaining agreement, or when further negotiations would take place.

Meanwhile, one new snag in negotiations between the league and the NFL Referees Association is the latter's request for a ratification bonus to approve a new deal in which the union has made concessions, sources familiar with the talks told ESPN's Chris Mortensen.

The league has resisted such a bonus because it believes it is a mechanism for the locked-out officials to receive money for games they have missed, the sources said. The union maintained to the NFL that a lump-sum bonus paid to it is a common treatment for its members to ratify an agreement in which it has made concessions to the league. Under the proposal, the union would have discretion to distribute the amount as it desires.

In the NFLPA's letter signed by 12 current or former players, the players ask why commissioner Roger Goodell issues suspensions and fines "in the name of player health and safety" while the regular officials the league entrusts to maintain health and safety are locked out.

"As players, we see this game as more than the 'product' you reference at times," the letter said. "You cannot simply switch to a group of cheaper officials and fulfill your legal, moral, and duty obligations to us and our fans. You need to end the lockout and bring back the officials immediately."

The NFL locked out the officials in June after their contact expired. It has used replacements for the first time since 2001, when the regular officials missed only the first week of action.

But those replacements had experience at the highest levels of college football, and the current replacements come from lower college levels or from other leagues such as Arena Football.

There have been numerous complaints by players and coaches about the officiating -- certainly more than when the regular officials work -- and one particularly embarrassing episode in which an official was removed from working a New Orleans game because he posted photos of himself in Saints gear on Facebook.

"The removal of the veteran officials from regular-season games left a group of your replacements who have proved to be incapable of keeping pace with the speed of the game," the players wrote. "Coaches and players have complained of numerous errors and failures including: erratic and missed calls on egregious holds and hits, increased skirmishes between players and confusion about game rules. Many replacements have lost control of games due to inexperience and unfamiliarity with players and rules."

Browns linebacker Fujita was suspended for three games by Goodell for his role in the Saints' bounty scandal, a suspension that was temporarily placed on hold by an appeals panel.

Among others signing the letter were Titans quarterback Matt Hasselbeck, Packers center Jeff Saturday and former star safety Brian Dawkins, now retired and currently an ESPN analyst.

Information from ESPN's Chris Mortensen and The Associated Press was used in this report.