US President Donald Trump has said his criticism of National Football League (NFL) players who kneel during the national anthem is not race-related. 

“The issue of kneeling has nothing to do with race,” he said in a tweet on Monday. “It is about respect for our Country, Flag and National Anthem. NFL must respect this!”

His comment echoed a similar remark he made the previous day. “This has nothing to do with race. I’ve never said anything about race,” Trump told reporters on Sunday.

“I think the owners should do something about it. It’s very disrespectful to our flag and our country,” he added.

On Friday, Trump set up a firestorm in the world of sports by using profanity to describe NFL players who kneel during the anthem, and then doubled down on his criticism in a series of follow-up tweets.

"Wouldn�t you love to see one of these NFL owners, when somebody disrespects our flag, to say, �Get that son of a b***h off the field right now, out, he�s fired,” Trump said at a rally in Alabama, adding that such an owner would be “the most popular person in this country.”

The president then called on NFL to change policy around the national anthem. 

The comments set the stage for mass protests along NFL sidelines. American football players across the US, including members of the Jacksonville Jaguars and the Baltimore Ravens, took part in mass protests during games on Sunday.

Trump�s tirade has intensified his spat with some of the biggest stars in US professional sports.

Players dropped to their knees or linked arms for the national anthem. Others stayed in their locker rooms during the national anthem ahead of NFL games.

On Saturday, NFL Commissioner Roger Goodell condemned Trump�s "divisive comments," and said they showed an "unfortunate lack of respect" for the NFL and its players.

The protest movement started last year when then-San Francisco 49ers quarterback Colin Kaepernick first refused to stand during the anthem in preseason.

"I am not going to stand up to show pride in a flag for a country that oppresses black people and people of color," he said at the time.

Since then, players in many teams have joined the protest in an effort to draw attention to racial injustice in the US, which has deteriorated under Trump.