Britain's UN ambassador said the United Kingdom "stands shoulder to shoulder with the United States" in tackling the nuclear threat from North Korea and making sure it can not pursue its nuclear and ballistic missile programmes.
A day earlier Trump warned the North against threatening the US.
"We always maintain a high state of readiness and have the capabilities to counter any threat, to include those from North Korea", Lt. Col. Christopher Logan, a Pentagon spokesman, told Business Insider. They evoked President Harry Truman's announcement of the U.S. atomic bomb dropped on Hiroshima, Japan, in 1945, in which he warned of "a rain of ruin from the air, the like of which has never been seen on this earth".
According to a report from the Defense Intelligence Agency, North Korea manufactured a miniaturized nuclear warhead, which can fit inside long-range missiles capable of reaching the U.S.
While the State Department is making every effort to resolve this global threat through diplomatic means, it must be noted that the combined allied militaries now possess the most precise, rehearsed and robust defensive and offensive capabilities on Earth, Mattis said.
Mattis' statements on Wednesday added to the increasingly heated rhetoric between North Korea and the United States.
In a statement, Mattis said the USA and its allies would win any arms race or conflict with North Korea.
North Korea said it was "carefully examining" a plan to strike Guam, which is home to about 163,000 people and a US military base that includes a submarine squadron, an airbase and a Coast Guard group.
Trump followed his threat with a tweet Wednesday declaring the U.S.is more than ready for a war.
A new report said that Pyongyang's nuclear program is progressing.
Tension in the region has risen since North Korea carried out two nuclear bomb tests past year and two intercontinental ballistic missile tests in July. The U.N. Security Council unanimously imposed new sanctions on North Korea on Saturday.
"What the president is doing is sending a strong message to North Korea in language that Kim Jong Un would understand, because he doesn't seem to understand diplomatic language", Tillerson said. Democratic leaders criticised Trump for his statement on North Korea.