US President Donald Trump signed into law the $717 billion annual defence policy bill, the earliest in the year the bill has become law in more than four decades.

"The National Defense Authorization Act (NDAA) is the most significant investment in our military and our warfighters in modern history, and I am very proud to be a big, big part of it," Trump said before signing the bill on Monday. "It was not very hard. You know, I went to Congress, I said let's do it, we got to do it. We're going to strengthen our military like never, ever before, and that's what we did," he said.