Trump made the remarks during a 20-minute phone conversation with South Korea's acting President and Prime Minister Hwang Kyo-ahn, Yonhap reported.
The U.S. president also spoke to Japanese Prime Minister Shinzo Abe by phone, calling Pyongyang's provocation a clear violation of United Nations Security Council resolutions.
According to a statement from the White House, Trump emphasized the pledge of defense between the United States and Japan, as well as the United States and South Korea.
He also stressed the importance of using all military capabilities to combat North Korea's ballistic missile threats.
The White House added Trump agreed on continued cooperation with Seoul and Tokyo in order to show North Korea the "very dire consequences" that could follow continued provocative and threatening behavior.
In his phone call with Abe, Trump described North Korea's threats as having entered a "new phase," and that the United States stood 100 percent with Tokyo, The Guardian reported.
"President Trump told me that the United States was with Japan 100 percent, and that he wanted his comments to be communicated to the Japanese people," Abe told reporters.
The Japanese prime minister had also told Trump Tokyo is willing to undertake a "large role and responsibility to enhance the deterrence provided by the alliance."
On Tuesday, North Korea said the missiles were tested in preparation for potential strikes against U.S. military bases in Japan, confirming an earlier hypothesis in Seoul that Pyongyang was targeting Japan bases from where reinforcements would arrive in the event of war.