US President Donald Trump has marked US Memorial Day and hailed the country's "greatest warriors", declaring "we owe them everything".

Trump said: "We gather today to honour the incredible service members who rest in glory in this cemetery and burial grounds around the world, and in a thousand lonely places known only to God in every hour of peril, in every moment of crisis.

"American warriors have left behind the blessings of home and family to answer their nation's call."