US President Donald Trump has ramped up his fight with the National Football League (NFL), calling on the popular league to ban players from kneeling in protest at games while the US national anthem is played.

Key points: Mr Trump says "the only way" is to set a rule preventing kneeling during the anthem

Mr Trump says "the only way" is to set a rule preventing kneeling during the anthem Hillary Clinton says Mr Trump's NFL spat is strategically targeted towards his fanbase

Hillary Clinton says Mr Trump's NFL spat is strategically targeted towards his fanbase The President has previously called for players who kneel to be fired

"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!" Mr Trump wrote on Twitter, fuelling his war of words with the multibillion-dollar NFL in his fifth-straight day of public comments on the issue.

League spokesman Joe Lockhart said he would not react to Mr Trump simply "exercising his freedom to speak".

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As far as sacking players who opt to kneel, Mr Lockhart refused to speculate.

"I will leave the hypotheticals and the speculation to others," he said.

"I'm not going to go down that road."

Earlier, President Trump praised two NFL teams that had largely steered clear of the controversy: the Arizona Cardinals — who linked arms and stood for the playing of the Star-Spangled Banner — and the Dallas Cowboys, who knelt before the song, but then proceeded to stand.

"The booing at the NFL football game last night, when the entire Dallas team dropped to its knees, was loudest I have ever heard. Great anger," Mr Trump wrote.

"But while Dallas dropped to its knees as a team, they all stood up for our National Anthem. Big progress being made-we all love our country!

"Ratings for NFL football are way down except before game starts, when people tune in to see whether or not our country will be disrespected!"

Television networks reported a mixed impact on viewership for Sunday's NFL games, in reference to Mr Trump's declarations of declining ratings.

The series of tweets were the latest salvo from Mr Trump after he ignited the fight with the players in the biggest-grossing American professional sports league last week.

On Friday, he told a political rally 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 is very strategic about who he attacks': Clinton

Sorry, this video has expired NFL players kneel during US anthem (Photo: Reuters)

Observers noted that Mr Trump's verbal assaults were likely to play well with his conservative base at a time when the Republican president is grappling with North Korea's nuclear threats, a humanitarian crisis in hurricane-struck Puerto Rico, an investigation into Russian meddling in the 2016 election, and a healthcare struggle in Congress.

Hillary Clinton, Mr Trump's ex-rival in the 2016 presidential contest, blasted the US President for targeting black players and stoking racial tensions.

"He's very strategic about who he attacks, and he is sending a message. It's a huge loud dog whistle to his supporters," Mrs Clinton told reporters.

Former San Francisco 49ers quarterback Colin Kaepernick first knelt during the national anthem last year, refusing to stand to protest police shootings of unarmed African-Americans.

Several players have made similar gestures in what they said is a call for social justice, not a slight against the country or its flag.

Critics, including Mr Trump, have said it is disrespectful, while proponents maintain that the protests embody the American right to free speech.

Sorry, this video has expired Donald Trump says his NFL comments have 'nothing to do with race'

Reuters/ABC