From left, San Francisco 49ers outside linebacker Eli Harold, quarterback Colin Kaepernick, center, and safety Eric Reid kneel during the national anthem before an NFL football game against the Dallas Cowboys in Santa Clara, California. File picture: Marcio Jose Sanchez/AP

Washington/ News York - US President Donald Trump ramped up his fight with the National Football League on Tuesday, calling on the league to ban players from kneeling in protest at games while the US national anthem is played.

"The NFL has all sorts of rules and regulations. The only way out for them is to set a rule that you can't kneel during our National Anthem!" Trump wrote on Twitter.

For the fifth straight day the president expressed public disapproval of the symbolic gesture, which has been adopted by some black players in the last year as a protest against racial disparities in the criminal justice system.

A spokesman for the NFL, the biggest-grossing US professional sports league, did not respond to requests for comment.

Trump praised two teams that played on Monday night and largely steered clear of the controversy. The Arizona Cardinals linked arms and stood for the playing of the "Star-Spangled Banner" along with the Dallas Cowboys, who knelt before the song.

Trump first berated the players last Friday, telling a political rally in Alabama that any protesting player was a "son of a bitch" who should be fired, and urged a boycott of NFL games, touching off protests by dozens of players, coaches and some owners before games on Sunday.

Trump's verbal assault may play well with his conservative base as the Republican president grapples a host of critical issues, including North Korea's nuclear threats, a humanitarian crisis in hurricane-struck Puerto Rico, an investigation into Russian meddling in the 2016 election and the healthcare struggle in Congress.

Baltimore Ravens players kneel down during the playing of the US national anthem before an NFL football game against the Jacksonville Jaguars. Picture: Matt Dunham/AP

Paul Ryan, the most senior Republican in the US House of Representatives, said he also disapproved of the gesture.

"People are clearly within their rights to express themselves how they see fit," Ryan told reporters on Tuesday. "My own view though is that we shouldn't do it on the anthem."

Hillary Clinton, Trump's Democratic rival in the 2016 presidential election, called Trump's repeated comments "a huge loud dog whistle to his supporters." "He's very strategic about who he attacks, and he is sending a message," she said in an interview with CBS.

Former San Francisco 49ers quarterback Colin Kaepernick first kneeled during the national anthem last year to protest police shootings of unarmed black men.

Current 49ers safety Eric Reid, who played with Kaepernick, wrote in a New York Times op-ed on Monday that they chose to kneel as a "respectful" gesture, comparing it to "a flag flown at half-mast to mark a tragedy."