U.S. President Donald Trump speaks before signing a memorandum "Increasing Access to High-Quality Science, Technology, Engineering and Math (STEM) Education" in the Oval Office of the White House in Washington, U.S., September 25, 2017. REUTERS/Joshua Roberts

(Reuters) - U.S. 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 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 denounced the symbolic gesture, which has been adopted by some black players in the last year to protest against racial disparities in the criminal justice system.

An NFL spokesman 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 "Star-Spangled Banner" along with the Dallas Cowboys, who knelt before the song.

Last Friday, Trump told a political rally any protesting player was a "son of a bitch" who should be fired, and urged a boycott of NFL games, triggering protests by dozens of players, coaches and some owners before Sunday's games.

Trump's verbal assault may appeal to his conservative base as the Republican president grapples with 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.

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

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

U.S. Attorney General Jeff Session told an audience at Washington's Georgetown Law School that the protesting athletes were wrong.

"The players aren't subject to any prosecution, but if they take a provocative act, they can expect to be condemned," he said.

Hillary Clinton, Trump's Democratic rival in the 2016 election, called Trump's comments "a huge, loud dog whistle to his supporters" in an interview with CBS.

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

His former teammate Eric Reid wrote in a New York Times opinion article that he and Kaepernick chose to kneel as a "respectful" gesture, comparing it to "a flag flown at half-mast to mark a tragedy."

(Reporting by Susan Heavey and Makini Brice in Washington and Jonathan Allen in New York; Editing by Jeffrey Benkoe and Jonathan Oatis)