"I'd be disappointed if something would be in the works, and if it is we will take care of it," Trump told reporters in the White House when asked about latest developments in the DPRK, the Xinhua news agency reported.
"North Korea, we are watching very closely," he added, without providing details.
Trump's remarks came days after Pyongyang carried out two "important tests" at its Sohae Satellite Launching Ground on December 7 and 13.
The DPRK's Academy of Defense Science said in a statement, carried Saturday by the official Korean Central News Agency (KCNA), that "the research successes being registered by us in defense science one after another recently will be applied to further bolster up the reliable strategic nuclear deterrent of the DPRK."
DPRK's Vice Foreign Minister Ri Thae Song also warned early December that Washington would soon need to decide what kind of "Christmas gift" it will receive from Pyongyang.
U.S. special envoy for the DPRK Stephen Biegun said Monday in Seoul during his meeting with South Korean President Moon Jae-in that the United States was unwavering in its efforts to denuclearize the Korean Peninsula via dialogue and diplomacy.
The DPRK and the United States held working-level denuclearization talks in Stockholm in October, but it ended without progress. Pyongyang set its deadline for the denuclearization negotiations at the end of this year.
( With inputs from IANS )