"The DPRK regime's actions will continue to be grossly overmatched by ours and would lose any arms race or conflict it initiates", he said.
North Korea said it was "carefully examining" a plan to strike Guam, which is home to about 163,000 people and a United States military base that includes a submarine squadron, an airbase and a Coast Guard group.
President Trump on Wednesday declared the U.S. nuclear arsenal "far stronger and more powerful than ever before" even as his top diplomat was working to calm the North Korea crisis and insisting there wasn't "any imminent threat".
He said the United States has a "very active, ongoing diplomatic effort" and that the country's "telephone lines remain open certainly to China and Russian Federation, as well as our allies".
In a series of early-morning tweets, Trump reaffirmed his threat from a day earlier by reposting the video of him warning that Pyongyang would be "met with fire and fury like the world has never seen" if it made more threats to the US.
Washington has warned it is ready to use force if needed to stop North Korea's ballistic missile and nuclear programs. "[Kim] has been very threatening beyond a normal state and as I said they will be met with fire and fury and frankly power the likes of which this world has never seen before".
On July 28, North Korea launched its second intercontinental ballistic missile (ICBM), which was the longest flight of a ballistic missile in the country's history, the Pentagon confirmed.
He was responding to reporters' questions on the North Korean nuclear capabilities. Democratic leaders criticised Trump for his statement on North Korea. "No matter how much the Kim regime deserves global condemnation and action - which it does - it is not a strategic or responsible response to issue wild threats of destruction", said House Democratic Whip Steny H Hoyer. A U.S strike against North Korea would likely result in a retaliatory strike against American ally South Korea, where thousands of American troops have been stationed since the end of the Korean War.
"We ought not engage in this sort of belligerent talk", he told CNN.