If North Korea carries out a nuclear test 'I would not be happy', Trump told the CBS television network's 'Face the Nation' program.
"And I can tell you also, I don't believe that the president of China, who is a very respected man, will be happy either", Trump said in excerpts of the interview released Saturday.
Asked if "not happy" signified "military action," Trump answered, "I don't know. I mean, we'll see."
North Korea test-fired a missile over the weekend in apparent defiance of a concerted US push for tougher international sanctions to curb Pyongyang's nuclear weapons ambitions.
The latest launch, which South Korea said was a failure, came just hours after US Secretary of State Rex Tillerson warned the UN Security Council of 'catastrophic consequences' if the international community - most notably China - failed to pressure the North into abandoning its weapons program.
Trump assailed the failed launch as a show of disrespect toward its ally China.
"North Korea disrespected the wishes of China & its highly respected President when it launched, though unsuccessfully, a missile today. Bad!" Trump wrote on Twitter.
North Korea is seeking to develop a long-range missile capable of hitting the US mainland with a nuclear warhead.
China, Pyongyang's number one trade partner, has repeatedly called for a return to talks on de-nuclearisation but has been reluctant to use economic pressure that could destabilise North Korea.