President Donald Trump is once again lashing out at football players who refuse to stand for the national anthem.

Trump claimed on Twitter Friday that "most of them are unable to define" what they're demonstrating against.

Instead, Trump tweets players should "Be happy, be cool!"

Numerous player demonstrations took place during the national anthem at several early NFL preseason games Thursday night. Players have been protesting police killings of black men, social injustice and racism.

Players including Miami Dolphins receiver Kenny Stills says he will continue to protest this season. He was joined by his teammate Albert Wilson Thursday night.

"Being part of this protest hasn't been easy. I thought I was going to be by myself out there, I had an angel with me, with Albert being out there. I'm grateful that he sees what's happening, he wants to stand up and do something about it as well." 

Trump wrote from his New Jersey golf resort that players "make a fortune doing what they love" and that those who refuse to stand "proudly" for the anthem should be suspended without pay.

Trump has told associates that he believes the anthem issue is a winning one that riles up his base.

NFL owners voted this summer to mandate all players stand for the national anthem or stay in the locker room.  That policy has since been put on hold while the league negotiates with the players union to find a solution that both sides can accept.

(The Associated Press, CBS News contributed to this report.)