File photo shows Princess Mako (R) and Kei Komuro attending a press conference in Tokyo on Sept. 3, 2017. (Kyodo)

TOKYO - Princess Mako, a niece of Japanese Emperor Naruhito, and her boyfriend Kei Komuro will marry by the year-end and may start a new life in the United States, but in a rare decision will not hold the usual related ritual ceremonies, a government source said Wednesday.