U.S. President Donald Trump kept up his verbal battle with the National Football League over players who drop to one knee during the national anthem, saying on Monday their acts of protest had nothing to do with racism.

Dozens of NFL players, coaches and even some owners joined in silent protest at games on Sunday against Trump's call for owners to fire players who do not stand during the "Star-Spangled Banner."

"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!" Trump said on Twitter.