National Football League ratings declined on Sunday in the US, though it was unclear whether the drop was related to President Donald Trump's call for fans to boycott the games because of player protests.

Match-ups on networks Fox and NBC drew fewer viewers than a year earlier, while CBS said the number of people tuning in rose. Monday night's game, featuring major draw Dallas Cowboys, may lift the weekend's ratings. Viewership for the entire season has been down.

Over the weekend, Trump criticised players for kneeling during the national anthem, a practice started by former San Francisco 49ers quarterback Colin Kaepernick as a protest against police brutality and mistreatment of black Americans. Trump said owners should fire players who protested and urged fans to skip watching NFL games.

Debate over the national anthem was "a side issue" that's likely to boost ratings for sports talks shows or pre-game shows, but not affect whether people watch the actual games, said Lee Berke, president of consultant LHB Sports Entertainment & Media.