The announcement Thursday warned that the North is preparing a plan to fire four of its Hwasong-12 missiles over Japan and into waters around the tiny island, which hosts 7,000 USA military personnel on two main bases and has a population of 160,000.
While Trump said the nuclear arsenal was more powerful than ever before, USA officials say it takes decades to actually modernize nuclear weapons, a move already under way under President Barack Obama's administration, and there are treaties that regulate nuclear expansion.
Tensions between the US and North Korea have been heightened in recent months, as the North accelerates the pace of its ballistic missile tests.
Mattis said in a statement that the United States and its allies would win any arms race or conflict with North Korea.
That's what some Twitter users, including actor and former Barack Obama aide Kal Penn, are demanding, after President Trump tweeted Friday morning that U.S.
The threat came hours after US President Donald Trump warned Pyongyang that any threat would be met with "fire, fury and frankly power the likes of which this world has never seen before".
Safe-Haven Currencies Rise as North Korea Tensions Brew
Wall Street saw a stock sell-off on Thursday as rising tensions between the USA and North Korea filled investors with worry. All 11 major S&P sectors were lower and the consumer discretionary index was the biggest loser with a 1 percent drop.
Tillerson spoke to reporters Wednesday after President Donald Trump and North Korea traded escalating threats of "fire", with the North saying it was examining plans for attacking Guam.
Furthermore, experts say it's nearly impossible that any initiative of the Trump Administration could already be having noticeable impact on the arsenal.
On Monday, two US B-1 bombers flew from Guam over the Korean peninsula as part of its "continuous bomber presence", a U.S. official said, in a sign of the island's strategic importance. "The only time our posture goes up is based on facts, not because of what Kim and Trump say to each other", the official added.
After noting President George W. Bush was also not successful, Trump said "Obama didn't even want to talk about it".
Former Republican Senator Gordon Humphrey, who represented New Hampshire in the United States Senate from 1979-1990, said in a letter to New Hampshire's congressional representatives that Trump has a "seriously sick psyche" and that his "sick mind and reckless conduct could consume the lives of millions".
Tens of thousands of United States troops remain stationed in South Korea and in nearby Japan, the only country to have been attacked with nuclear weapons.