NFL team owners are considering new rules that would require players to stand during the anthem: Getty

The NFL is reportedly considering amending its rules to require players to stand during the national anthem.

An spokesman for the league said that owners are planning on discussing the potential change next week, after weeks of conflict with the White House, Dow Jones reports.

Players began kneeling last season to protest police violence and racial inequality in the United States, led by former San Francisco 49ers quarterback Colin Kaepernick. That protest movement has picked up steam recently after Donald Trump criticised the league for allowing players to do so, saying that they should fire the protesting players. After that initial criticism, a wave of players joined in the protest, heating up the conflict.

Mr Trump appeared to threaten the league just hours before news broke that the owners were considering changing their rules, saying that the league gets massive tax breaks, and pondering making changes to tax laws.

"Why is the NFL getting massive tax breaks while at the same time disrespecting our Anthem, Flag and Country?" Mr Trump tweeted. "Change tax law!"

An NFL spokesperson had responded, however, noting that the league hasn't been a tax exempt organisation since 2015.

Even before then, though, only a relatively small portion of the NFL was listed as tax exempt. The league sees revenues around $10 billion a year, but only its main office was tax exempt in the past. That office only generated $9 million in tax-exempt revenue in 2014.

The individual teams, on the other hand, all pay federal taxes on all of their revenues. That includes revenue from ticket sales, merchandise, sponsorships, and from broadcast rights.

It's not clear what Mr Trump could actually do from the White House to hurt the league's bottom line aside from waging the cultural ware he has already started. The league does receive significant tax breaks to build things like stadiums, but that is largely a local matter, impacted primarily by state and municipal laws that address those issues.