NFL team owners will consider requiring football players to stand for the U.S. national anthem after President Donald Trump on Tuesday suggested using tax laws to penalise the league for players who kneel in protest of racial injustice.

Trump, a Republican, escalated his feud with the National Football League in a Twitter post asking if the league should get tax breaks while some athletes kneel in protest when the “Star-Spangled Banner” is played at the start of each game.

“Why is the NFL getting massive tax breaks while at the same time disrespecting our Anthem, Flag and Country? Change tax law!” Trump wrote on Twitter.

The world’s top-grossing sports league gave up its tax-free status two years ago. Its owners are preparing to address the anthem issue at their meeting in New York between Oct 17-18, NFL chief spokesman Joe Lockhart told reporters on a conference call on Tuesday.

“Everyone at this point is frustrated by the situation,” Lockhart said. “The commissioner and the owners do want the players to stand. We think it is an important part of the game."