U.S. President Donald Trump has said Japan should have shot down the North Korean missiles that flew over the country before landing in the Pacific Ocean earlier this year, diplomatic sources have said, despite the difficulties and potential ramifications of doing so.

The revelation came ahead of Trump's arrival in Japan on Sunday at the start of his five-nation trip to Asia. Threats from North Korea's nuclear weapons and missile development programs were set to be high on the agenda in his talks with Prime Minister Shinzo Abe on Monday.

Trump questioned Japan's decision not to shoot down the missiles when he met or spoke by phone with leaders from Southeast Asian countries over recent months to discuss how to respond to the threats from North Korea, the sources said.