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.

The U.S. president said he could not understand why a country of samurai warriors did not shoot down the missiles, the sources said.

In defiance of international sanctions imposed to compel Pyongyang to abandon its nuclear weapons and missile development programs, North Korea test-launched ballistic missiles on Aug. 29 and Sept. 15 that flew over Hokkaido before falling into the Pacific Ocean.