A 97-year-old US Second World War veteran has publicly 'taken the knee' in support of NFL players protesting during the US national anthem after President Donald Trump called for those who did so to be sacked.

John Middlemas, who served in the US Navy for 21 years as a submariner, made the gesture in a photo posted by his grandson on Twitter saying "those kids have every right to protest".

The veteran’s intervention comes after scores of NFL players took a knee at the weekend as the US national anthem was played.