US President Donald Trump on Friday criticized protestors who resorted to kneeling down during the national anthem after NFL quarterback Drew Brees apologized for remarks he made about the practice.

George Floyd's death on May 25 -- at the hands of a white police officer who knelt on his neck for almost nine minutes -- has become the latest flashpoint for rage over police brutality against African Americans, propelling the issue of race to the top of the political agenda ahead of the US presidential election on November 3.

To this end, the kneeling pose, popularised by NFL quarterback Colin Kaepernick, has become a symbol of the fight for racial justice in the United States.

Brees said this week he would "never agree with anybody disrespecting the flag," referring to the possibility of players kneeling during the "Star-Spangled Banner" in the upcoming NFL season. Brees apologized on Thursday, saying his words "lacked awareness and any type of compassion or empathy."

Trump tweeted on Friday that Brees "should not have taken back his original stance."

"We should be standing up straight and tall, ideally with a salute, or a hand on heart," Trump wrote. "There are other things you can protest, but not our Great American Flag - NO KNEELING!"

The kneeling pose has been seen at protests in cities across the country in the wake of the death of George Floyd, an unarmed black man, while in police custody in Minneapolis.

Brees' initial remarks angered top athletes, who objected to the equating of the protest with disrespecting the American flag.

"You literally still don't understand why Kap was kneeling on one knee??" NBA great LeBron James posted on Twitter earlier this week. "Has absolute nothing to do with the disrespect of Flag of the United States and our soldiers(men and women)."