US Defense Secretary Jim Mattis on Saturday threatened North Korea with "a massive military response" should it decide to use nuclear weapons.
Mattis said that the threat of a nuclear attack from North Korea was accelerating, but that it was still no match for US and South Korean firepower.
"North Korea has accelerated the threat that it poses to its neighbors and the world through its illegal and unnecessary missile and nuclear weapons programs," he said during his second day in South Korea for annual defense talks.
Read more: Jim Mattis says US goal not war with North Korea
Mattis said diplomacy remained a "preferred course of action" but stressed, "our diplomats are most effective when backed by credible military force."
"Make no mistake any attack on the United States or our allies will be defeated."
Increasing missile payload
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The US would not accept the North as a nuclear power, he underscored at a joint news conference with his South Korean counterpart Song Young-moo.
Song told journalists that he and Mattis had agreed to cooperate further on strengthening Seoul's defense capabilities. He said measures would include lifting warhead payload limits on South Korean conventional missiles and supporting the country's acquisition of "most advanced military assets."
He refused to answer questions on nuclear-powered submarines though, which some South Korean officials have been calling for.
Will not redeploy nuclear weapons
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Some conservative politicians in the South have also been calling for the redeployment of US tactical nuclear weapons on the Korean Peninsula, but Mattis and Song were both dismissive of the idea.
"When considering national interest, it's much better not to deploy them," said Song, saying allies already had "sufficient means" to respond to a North Korean nuclear attack.
The two defense leaders also discussed the terms under which the South would be given wartime operational control of its forces. Under current agreements, the South's forces would operate under the US-led United Nations Command in the event of war with the North.
US President Donald Trump will make his first presidential visit to South Korea next week as part of his Asia tour, which also includes Japan, China, Vietnam and the Philippines. He is expected to use the visit to address concerns about North Korea, to speak to Seoul's parliament and to visit a US military base there.
aw/ng (AP, Reuters, AFP, dpa)