After his volley of verbal attacks on black athletes led players across the country to kneel during the US national anthem over the weekend, the besieged president played defense on Twitter.

Trump had started the furor by attacking players like Colin Kaepernick -- who first took a knee through renditions of the "Star-Spangled Banner" during last year's American football season to protest police brutality toward African Americans -- as a "son of a bitch" who should be fired.

In a separate feud, Trump also disinvited basketball superstar Stephen Curry from a White House event.

More than 150 pro football players took a defiant stance on Sunday, kneeling, linking arms or raising clenched fists during the anthem before 14 games.

In response, the US leader doubled down on those remarks by urging fans to boycott the NFL as long as the protests continued.

And on Monday, keeping the issue alive for a fourth day, Trump insisted: "The issue of kneeling has nothing to do with race. It is about respect for our Country, Flag and National Anthem. NFL must respect this!"

Twelve hours later, he was still at it, denying any rifts about the issue with his influential chief of staff.

"General John Kelly totally agrees w/ my stance on NFL players and the fact that they should not be disrespecting our FLAG or GREAT COUNTRY!"

Trump -- who faces low poll numbers and is struggling to enact his agenda -- earlier tried to single out the NFL players who protested.

"Many people booed the players who kneeled yesterday (which was a small percentage of total). These are fans who demand respect for our Flag!" he tweeted.

He also pointed to the Pittsburgh Penguins' acceptance of a White House invitation and support from racecar fans.

"So proud of NASCAR and its supporters and fans. They won't put up with disrespecting our Country or our Flag - they said it loud and clear!" Trump tweeted.

Only a handful of NASCAR drivers have been African American.

But driver Dale Earnhardt also took to Twitter, implicitly rebuking Trump with a quote from former president John F Kennedy: "All Americans R granted rights 2 peaceful protests Those who make peaceful revolution impossible will make violent revolution inevitable."

Trump also drew a furious backlash from NBA basketball stars, including superstar LeBron James, who described the president as a "bum."

"He doesn't understand how many kids, no matter the race, look up to the president of the United States for guidance, for leadership, for words of encouragement," said James.

"The people run this country. Not one individual. And damn sure not him."

The White House denied that Trump's "son of a bitch" remarks were unbecoming of his office.

"I think that it's always appropriate for the president of the United States to defend our flag, to defend the national anthem, and to defend the men and women who fought and died to defend it," said spokeswoman Sarah Huckabee Sanders.

"The president is not talking about race," she claimed. 

- Courting controversy -

Trump has wholeheartedly embraced the controversy, with his advisors suggesting it plays well with his largely white base.

Trump also changed his Twitter background photo to an American flag and stated that the "White House never looked more beautiful than it did returning last night."

That has led critics to accuse Trump of creating a diversion.

His efforts to repeal Barack Obama's health care reforms have run aground and would-be signature tax reforms are giving way to much less ambitious tax cuts.

At the same time, Trump faces a number of challenges from overseas, not least a war of words with North Korea that threatens to become a shooting war.

Trump has also faced criticism for his low-profile White House response to Hurricane Maria, which has left much of the US island territory of Puerto Rico. (**)