Vientiane, Laos: President Barack Obama weighed in Monday on the controversy surrounding Colin Kaepernick, saying the San Francisco 49ers quarterback was exercising his "constitutional right" to protest the US national anthem.

Kaepernick has drawn fierce criticism for refusing to stand during renditions of "The Star Spangled Banner", and was roundly booed during a match in San Diego last week in which he knelt during the song.

Obama, speaking on the sidelines of the G20 Summit in China, said Kaepernick had succeeded in drawing attention to "topics that need to be talked about."

Kaepernick has said his protest is a statement against racial injustice and police brutality.

"My understanding, at least, is that he's exercising his constitutional right to make a statement," Obama said. 

"I think there's a long history of sports figures doing so. I think there are a lot of ways you can do it.

"As a general matter, when it comes to the flag, and the national anthem, and the meaning it holds for our men and women in uniform and those who fought for us, that is a tough thing for them to get past, to then hear what his deeper concerns are. 

"But I don't doubt his sincerity, based on what I've heard. I think he cares about some real, legitimate issues that have to be talked about. And if nothing else, what he's done is he's generated more conversation around some topics that need to be talked about."

Obama added that Kaepernick's protest may be defined as "messy" but said "that's the way democracy works." 

"I'd rather have young people who are engaged in the argument and trying to think through how they can be part of our democratic process than people who are just sitting on the sidelines not paying attention at all," Obama said.