U.S. President Donald Trump on Sunday stepped up his criticism of National Football League players who protest during the pre-game national anthem, calling on fans to consider boycotting the NFL if teams do not discipline them.

In an early morning post on Twitter, Trump pressed ahead with his feud about players who refuse to stand for the anthem in what began last year as a form of protest over excessive use of force by police against African-Americans.

"If NFL fans refuse to go to games until players stop disrespecting our Flag & Country, you will see change take place fast," Trump wrote on Sunday. "Fire or suspend!"

In a second tweet, Trump, who is spending the weekend at his golf club in Bedminster, New Jersey, said that the "league should back" fans who are upset about the protests.

In 2016, then San Francisco 49ers quarterback Colin Kaepernick put one knee to the ground during pre-game renditions of the "Star Spangled Banner". Several players have since made similar gestures of protest before games.

At a political rally in Alabama on Friday, Trump suggested any protesting football player was a "son of a bitch" and should lose his job.

On Sunday morning, New England Patriots Chairman and CEO Robert Kraft, who has dined with Trump and who the president considers a friend, criticized Trump and defended players' right to protest.

"I am deeply disappointed by the tone of the comments made by the president on Friday," Kraft said on a statement posted on Twitter. Kraft said he supports his players' "right to peacefully affect social change and raise awareness in a manner they feel is most impactful."

NFL Commissioner Roger Goodell said Trump's comments in Alabama revealed an "unfortunate lack of respect" for the NFL and its players.

As commissioner, Goodell reports to NFL owners, some of whom have supported Trump. New York Jets owner Woody Johnson, a major Trump presidential campaign donor, was confirmed by the Senate last month as Trump's pick to serve as U.S. ambassador to Britain.

The union representing professional football players also rejected Trump's comments, saying it would defend their right to freedom of expression.

'INVITATION WITHDRAWN!'

National Basketball Association players also struck back against comments by the president on Saturday after Trump clashed with one of the biggest stars in the NBA.

In an early morning Twitter message on Saturday, the president rescinded a White House invitation to Stephen Curry, who had said he would "vote" against the planned visit by the NBA champion Golden State Warriors.

"Going to the White House is considered a great honour for a championship team. Stephen Curry is hesitating, therefore invitation is withdrawn!" Trump tweeted.

Curry told a news conference in Oakland, California: "It's beneath the leader of a country to go that route."

The Oakland-based Golden State Warriors said in a statement the team had intended to meet to discuss the potential visit at the first opportunity on Saturday morning.

Singer Stevie Wonder appeared to evoke protests by Kaepernick and other athletes when he put one knee to the stage during a concert at the Global Citizen Festival in New York City on Saturday.

"Tonight, I'm taking a knee for America," Wonder told the audience as his son, Kwame Morris, helped him down.

Wonder then put his other leg down so that he was kneeling and facing the cheering crowd, with his son doing the same.

"I’m taking both knees," Wonder said. "Both knees in prayer for our planet, our future, our leaders of the world and our globe."