The NFL must “get tough and smart” over the national anthem protest because it is “killing” the league, says US president Donald Trump.

Trump has criticised players who kneel for the anthem, a move started by Colin Kaepernick in 2016, to highlight racial injustice and police brutality.

Some NFL owners reportedly believe players could soon be kept in locker rooms during the anthem as a result.

“That’s almost as bad as kneeling,” Trump wrote on Twitter.

“When will the highly paid Commissioner finally get tough and smart? This issue is killing your league!

In October, NFL Commissioner Roger Goodell wrote to franchises stating the take the knee movement threatened “to erode the game’s unifying power”.

Kaepernick, 30, has been without a team since he opted out of his contract with the San Francisco 49ers in March and has filed a grievance against team owners.

He first protested by sitting during the anthem before opting to kneel.