"I just want everybody to understand and fully know that the United States of America stands behind Japan, its great ally, 100 percent", Trump said. The missile was launched at Panghyon from North Phyongan Province at 0755 KST traveled around 500km after a rapid vertical ascent of 550km.
The UN Security Council (UNSC) will hold a closed meeting on the launch at 5PM EST.
North Korea also regards all such sanctions as a violation of its sovereignty and right to self-defence.
South Korea's Foreign Ministry issued the statement in response, saying the missile test was a "blatant and obvious" violation of existing UN Security Council resolutions and a "serious threat" to global security.
"Obviously, North Korea is a big, big problem and we will deal with that very strongly", Mr. Trump said. Mattis later stopped in Japan, in what was his first overseas trip as head of the Defense Department, to reaffirm the US security commitment to the Asia Pacific allies.
The North Korean regime said that it had launched a medium to long-range ballistic missile on Sunday.
He condemned joint military exercises carried out annually by South Korea and the United States, as well as what he called "nuclear threats" and blackmail towards his country.
Though Pyongyang has been relatively quiet about the transfer of power to the Trump administration, its state media has repeatedly called for Washington to abandon its "hostile policy" and vowed to continue its nuclear and missile development programs until the USA changes its diplomatic approach.
But foreign analysts say North Korea has yet to develop warheads small enough to be loaded on a long-range missile created to hit targets like the US mainland.
The claim came as U.S. Ambassador to the United Nations Nikki Haley called on the U.N.to respond with unspecified "actions".
US, Japanese and South Korean military officials held a teleconference on Monday in which they condemned the launch as "a clear violation" of multiple security council resolutions. Below is a photo he took of Trump and Abe reviewing proposed responses to North Korea's test under the moonlight as staffers used their cell phones to make the intelligence recommendations readable.
North Korea yesterday rejected the United Nations Security Council's statement on its weekend missile launch, and declared that all its tests were "self-defence measures" created to protect its people. In response, the United States plans to deploy a missile defense system known as Thaad in South Korea, a move opposed by China, North Korea's primary ally. However, the spokesperson added that the USA "remain vigilant in the face of North Korean provocations and are fully committed to working closely with our Republic of Korea and Japanese allies to maintain security".
The missile was the first test since Donald Trump took office on January 20.