President Donald Trump has called for NFL players who kneel during the national anthem to be suspended without pay, renewing his attack on the protesting athletes.
"The NFL players are at it again - taking a knee when they should be standing proudly for the National Anthem," Mr Trump wrote on Twitter on Friday.
"A football game, that fans are paying soooo much money to watch and enjoy, is no place to protest. Most of that money goes to the players anyway.
"Find another way to protest. Stand proudly for your National Anthem or be Suspended Without Pay!"
The NFL players are at it again - taking a knee when they should be standing proudly for the National Anthem. Numerous players, from different teams, wanted to show their “outrage” at something that most of them are unable to define. They make a fortune doing what they love......
— Donald J. Trump (@realDonaldTrump) August 10, 2018
Mr Trump's salvo came after Miami players Kenny Stills and Albert Wilson took a knee during a rendition of "The Star-Spangled Banner" on Thursday.
In a separate protest, Philadelphia Eagles players Malcolm Jenkins and De'Vante Bausby raised their fists during the playing of the song before their game against the Pittsburgh Steelers.
Former San Francisco 49ers quarterback Colin Kaepernick launched the protest movement in 2016, declining to stand for the national anthem to draw attention to racial and social injustice following the deaths of several unarmed black men during confrontations with law enforcement.
NFL players – and a baseball star – 'take a knee for America'
The protests had largely fizzled out until Mr Trump last year described Kaepernick and other players who took a knee during the anthem as "sons of bitches" who should be fired.
Political analysts have predicted Mr Trump would seek to keep the issue alive to energise supporters ahead of November's crucial mid-term elections, framing protesters as unpatriotic and disrespectful to the military.
New anthem policy
The anthem controversy has caused deep divisions across the NFL, which oversees the most popular sport in the United States.
NFL owners approved a new national anthem policy earlier this year that requires players to stand if they are on the field during the anthem, and fines a team for any protest by its players.
The NFL decided to put a temporary hold on the implementation of that policy while it meets with the players' union to find a solution.
Several team owners had also stated they would not impose fines on players who protested.
Meanwhile, Kaepernick tweeted support for Miami duo Stills and Wilson on Thursday.
My brother @kstills continued his protest of systemic oppression tonight by taking a knee. Albert Wilson @iThinkIsee12 joined him in protest. Stay strong brothers!✊��
— Colin Kaepernick (@Kaepernick7) August 10, 2018
The NFL issued a statement Thursday night saying it will continue to play the anthem before games.
"There has been no change in the NFL's policy regarding the national anthem," league spokesman Brian McCarthy said.
"The anthem will continue to be played before every game, and all player and non-player personnel on the field at that time are expected to stand during the presentation of the flag and performance of the anthem. Personnel who do not wish to do so can choose to remain in the locker room."