The US vice president has walked out of an American football game after some players knelt during the national anthem, a protest against perceived racial injustice that is becoming a growing row.

Mike Pence tweeted that "I will always stand for our flag and national anthem".

A number of NFL players have been kneeling in protest against police brutality against African Americans.

The row escalated last month when President Donald Trump called for players who knelt during the anthem to be fired, and urged fans to boycott games.

At the game Mr Pence and his wife Karen were attending, more than a dozen players from the San Francisco 49ers knelt during the anthem.

Some of Mr Pence's home team, the Indianapolis Colts, also linked arms in a show of solidarity.

Mr Pence used the opportunity to back his boss. "I stand with President Trump," he wrote on Twitter.

The protest begun last year with San Francisco quarterback Colin Kaepernick did not stand during the national anthem.

The act of kneeling during the anthem has enraged the right-wing in the US, but shows little sign of abating.