US Secretary of Defense Jim Mattis has warned North Korea of a "massive military response" to any use of nuclear weapons as tensions remain high ahead of US President Donald Trump's visit to South Korea.
He warned Pyongyang that its military was no match for the US-South Korean alliance, and that diplomacy was most effective "when backed by credible military force".
"Make no mistake - any attack on the United States, or our allies, will be defeated. And any use of nuclear weapons by the North will be met with a massive military response that is both effective and overwhelming," Mr Mattis said.
He said he could not imagine the United States ever accepting a nuclear North Korea, warning that its rapidly advancing nuclear and missile programmes would undermine, not strengthen, its security.
Mr Mattis has been stressing during his week-long trip to Asia that diplomacy is America's preferred course, a message he returned to after top-level military talks in Seoul and at the tense border area with North Korea yesterday.
Mr Mattis' South Korean counterpart, Defense Minister Song Young-moo, dismissed the idea of deploying tactical nuclear weapons to the peninsula as a response to the North's advances.
But the US-Korean alliance had the ability to respond, even in the event of a nuclear attack from the North, Mr Song added.
Tension between North Korea and the United States has been building after a series of nuclear and missile tests by Pyongyang and verbal exchanges between North Korean leader Kim Jong Un and Mr Trump.
The CIA has said North Korea could be only months away from developing the ability to hit the United States with nuclear weapons, a scenario Mr Trump has vowed to prevent.
Mr Mattis, too, said the North Korean leader’s behaviour had left no room to imagine accepting Pyongyang's nuclear status.
"I cannot imagine a condition under which the United States can accept North Korea as a nuclear power," he told a news conference.
Mr Trump - who has threatened to destroy North Korea if necessary - leaves on his first trip as president to Asia next week, including a stop in South Korea to meet President Moon Jae-in.
After talks with Mr Mattis yesterday, President Moon said the "aggressive deployment" of US strategic assets in the region, which have included over flights by US bombers, had been effective in deterring the North Korean threat.
US intelligence experts said Pyongyang believes it needs the nuclear weapons to ensure its survival and have been sceptical about diplomatic efforts, focusing on sanctions, to get Pyongyang to denuclearise.
Mr Mattis suggested however, that Pyongyang needed to understand that its weapons programmes would not strengthen its defences.
The North said it wants a nuclear-tipped missile capable of reaching the United States.
"If it remains on its current path of ballistic missiles and atomic bombs, it will be counter-productive," he said, adding North Korea would be "reducing its security".
During his trip to the inter-Korean Demilitarised Zone (DMZ) yesterday, Mr Mattis was briefed on the posture of North Korean artillery.
Mr Song suggested that US and South Korean missile defences simply could not stop all of the weapons trained on the South.
"Defending against this many LRAs (long-range artillery) is infeasible in my opinion," Mr Song told Mr Mattis at the DMZ, citing a need for strategies to "offensively neutralise" the artillery in the event of a conflict.
Mr Mattis replied: "Understood."
Still, Mr Mattis reaffirmed to reporters in South Korea that military options do exist to deal with the North Korean threat that spare the South's capital, Seoul.
He did not disclose what those options were.
