Donald Trump's remarks come ahead of his first visit to Asia since taking office, with soaring tensions over North Korea's nuclear and missile programs expected to take center stage.
"Japan is a warrior nation, and I tell China and I tell everyone else that listens, I mean, you're gonna have yourself a big problem with Japan pretty soon if you allow this to continue with North Korea," Mr Trump said on Fox News.
But Mr Trump also said President Xi Jinping has been "pretty terrific" on North Korea and "China is helping us".
After a chiding from Mr Trump for failing to rein in Kim, China has implemented tougher UN sanctions against North Korea and Mr Xi's relationship with the US leader appears to be warming.
North Korea in July launched two intercontinental ballistic missiles apparently capable of reaching the US mainland - described by its leader Kim Jong-Un as a gift to "American bastards".
The North followed that up with two missiles that passed over Japan, and its sixth nuclear test, by far its most powerful yet.
The US president raised the specter of Japan taking action over North Korea as Prime Minister Shinzo Abe continues efforts to change its US-imposed pacifist constitution, seen by conservatives as an outdated legacy of wartime defeat, so Japan can formally transform its well-equipped and well-trained Self Defense Forces into a full-fledged military.
Mr Trump has warned of "fire and fury" in response to Pyongyang's threats, and derisively dubbed Kim "Rocket Man", who responded by calling him a "dotard".
