Colin Kaepernick

Kaepernick, 30, has not played in the NFL since opting out of his contract with the San Francisco 49ers in 2017, following a season in which he was both praised and criticised for kneeling during the national anthem in protest at perceived police brutality and racial injustice.

But Kaepernick and his protest have remained in the public eye and became the face of Nike's latest "Just Do It" advertising campaign last month, for which he was later backed by James.

The San Francisco 49ers quarterback started the protest last month after he sat during the national anthem for a preseason game, saying he would not "show pride in a flag for a country that oppresses black people and people of color."

Since then, other professional athletes have also chosen not to stand for the national anthem, sparking a national debate.

Kaepernick, a San Francisco 49ers quarterback, declined to stand during the playing of the national anthem ahead of a pre-season game on Aug. 26 and a game last week, prompting both outrage and support from NFL players and fans across the country.

On a list of all jerseys available on NFLShop.com, Kaepernick's was first when arranged by "Top Sellers," surpassing the jerseys for high-profile rookie players Carson Wentz of the Philadelphia Eagles and Ezekiel Elliott of the Dallas Cowboys.

Speaking in China, Mr Obama said the player had raised legitimate issues.

Mr Kaepernick stirred controversy when he sat during the national anthem to protest against racial injustice.

Some players have followed his example and sat or kneeled through the anthem.

Asked about the issue during a news conference at the G20 in Hangzhou, President Obama said it was tough for those in the military to understand why Mr Kaepernick might snub the national anthem.

But he said he did not doubt his sincerity to raise issues.