WASHINGTON – North Korean leader Kim Jong Un “should take heed” and stop the nuclear threats that could lead to the destruction of his people, Secretary of Defense Jim Mattis said Wednesday.

North Korea must “stand down its pursuit of nuclear weapons,” Mattis warned Wednesday in a prepared statement.

“The [North Korean] regime’s actions will continue to be grossly overmatched by ours and would lose any arms race or conflict if it initiates,” Mattis said.

It was Mattis’ first public comments on North Korea since President Donald Trump on Tuesday threatened “fire and fury” if North Korea does not stop its increasingly threatening tests of both intermediate range and intercontinental ballistic missiles.

Mattis’ statement struck a more bellicose tone than usual, highlighting the increased pressure and risk the U.S. is under.

On Tuesday several news outlets confirmed that the intelligence community now believes that North Korea has the ability to miniaturize a nuclear warhead which, matched up with the projected capabilities North Korea has shown in its two intercontinental ballistic missile launches, has put the U.S. mainland at risk.

On Tuesday in response to Trump’s threat, Kim Jong Un responded by issuing a direct threat to Guam, the U.S. island territory in the Pacific Ocean that is about 2,100 miles from North Korea – and well within the range of the missile capabilities North Korea has shown so far.

Guam is home to two major U.S. military installations, Andersen Air Force Base and Naval Base Guam, and approximately 16,400 military personnel and dependents. B-1s, B-52s and B-2 bomber aircraft are on regular rotation at Andersen; Naval Base Guam serves as the home port of several Los Angeles-class attack submarines, armed MH-60 helicopters and auxiliary ships.

It also has the most robust ground-based missile defense capabilities in the region. The Pentagon permanently deployed the Terminal High Altitude Area Defense system, also known as THAAD, to Guam in April 2016 and maintains an AN/TPY-2 radar at Andersen.

A recent test in Alaska showed the system was capable of intercepting a North Korean intermediate range ballistic missile, said Riki Ellison, president of the Missile Defense Advocacy Alliance.

Pentagon spokesman Army Col. Rob Manning said in response to the threat issued by North Korea, no changes had been made to Guam’s force protection levels or force posture.

Trump on Wednesday said on Twitter that he plans to modernize the U.S. nuclear force.  

“Hopefully we will never have to use this power, but there will never be a time that we are not the most powerful nation in the world!” Trump wrote on Twitter. 

Secretary of State Rex Tillerson, who was already traveling in the region, made a surprise stop at Guam Wednesday. Tillerson told reporters traveling with him that he thought the tweets sent a “clear message” to North Korea.

“The president was reaffirming that the U.S. has the capability to fully defend itself ... and we will do so,” Tillerson said.