Mr Mattis also warned Pyongyang that North Korea’s military was no match for the US-South Korean alliance and that diplomacy was most effective “when backed by credible military force”.
The Defence Secretary said: “Make no mistake – any attack on the United States, or our allies, will be defeated.
“And any use of nuclear weapons will be met with a massive military response that is both effective and overwhelming.”
Tensions between North Korea and the United States has been building after a series of nuclear and missile tests by Pyongyang.
The CIA has warned North Korea could be only months away from developing the ability to reach the United States with nuclear weapons.
President Trump who has threatened to destroy North Korea if necessary is heading to Asia this week, with his trip including a stop in South Korea to meet President Moon Jae-in.
More to follow…