NFL owners continue to push for an expanded season even as players continue to demonstrate little appetite for such an idea.

Sources who have been briefed on the current state of CBA negotiations told ESPN that there has been little substantive progress in talks so far. The players' side is determined to up its share of the revenue pool in the next CBA, while the owners so far have indicated that they're not interested in discussing the revenue split absent a discussion on the possibility of expanding the season.

No concrete proposals have been submitted by the owners' side regarding the specifics of an expanded season, the sources said. Ideas that have been discussed include expanding the regular season to 18 games, expanding the regular season to 17 games or simply adding an extra round of playoffs. All of those concepts would include a reduction in the number of preseason games -- an idea both sides favor.

Currently, 30 of the 32 teams play four preseason games each year (two get selected to play in a fifth preseason game in connection with the Hall of Fame induction ceremonies), and even the commissioner has in the past described the preseason as a substandard product. But since they are sold as part of season-ticket packages, preseason games do represent a revenue source for teams, and they don't want to give them up without adding regular-season or postseason games in exchange.

To this point, it's unclear what kind of regular-season or postseason modification the owners ultimately will seek. Discussions in recent weeks with sources on the ownership side have indicated that owners are split on how to expand the season, and while some want to push for 18 games, others would be more comfortable with expanding the playoff field instead. Expanded playoffs are an idea that sources on the players' side have indicated they would consider.

The players, however, don't appear to be as interested in regular-season expansion, and one source familiar with the talks said Andrew Luck's retirement announcement Saturday night has had an effect on their mindset. Players already were disinclined to consider an expansion of the season, and days after one of the game's brightest stars walked away at age 29 because of the physical toll from playing the game, the players' appetite for more games is even lower.

One owner, the Falcons' Arthur Blank, advocated regular-season expansion but said safety concerns would first need to be addressed.

"In terms of expanding the length of the season -- whether it be with additional playoff games or regular-season games and whether it would be to 17 or 18 -- I certainly would be in favor of an expansion, probably initially to 17 games to see how that works, then have 18," Blank told ESPN's Vaughn McClure. "But -- and the but is the biggest caveat here -- if we do that, we have to make sure that our first priority, which is player safety and health, is being kept in the forefront of that discussion.

"So maybe additional consideration of players on the roster. It may mean additional practice-squad players. There are a number of things you have to do to ensure that we want to protect our players. I would say that's of prime importance. We shouldn't consider expanding the season without having reasonable discussions to those concerns."

It's possible the owners could offer a generous enough financial concession to get the players to consider an expansion of the season, but to this point there has been no movement on the big-picture financial aspect of the negotiations -- i.e., the revenue split between players and owners. The current collective bargaining agreement, signed in 2011, stipulates that the players' share of all league revenue may not fall below 47 percent in any year of the deal. The deal expires in March 2021.

Owners and players met Monday night and were scheduled to meet again Tuesday in Chicago -- the second week in a row they have held face-to-face discussions. The schedule for further negotiations is unclear and could depend on what progress is made Tuesday. The owners have expressed a desire to have a new agreement in place before the start of the 2019 season, but since that is now only nine days away, optimism on that front appears to be on the wane.