Defense Secretary Ash Carter warned today that any attack on American allies or use of nuclear weapons by North Korea would be met with an "overwhelming" US response.
He was speaking at the start of talks between the top US and South Korea diplomats and defense officials to discuss their response to the growing threat from North Korea's nuclear and missile programs.
The meeting comes weeks after the North's most powerful nuclear test explosion to date, and days after its failed test launch of a ballistic missile.
On the agenda is how to strengthen the so-called "extended deterrence" provided by US nuclear forces in defending South Korea.
Seoul is looking to allay calls from conservatives at home who want South Korea to develop its own nuclear arsenal.
"The US commitment to the defense of South Korea is unwavering. This includes our commitment to provide extended deterrence, guaranteed by the full spectrum of US defense capabilities," Carter said in opening remarks.
"Make no mistake, any attack on America or our allies will not only be defeated, but any use of nuclear weapons will met with an overwhelming and effective response," he said.
The talks are between Secretary of State John Kerry and Carter, and their South Korean counterparts Foreign Minister Yun Byung-se and Defense Minister Han Min-koo.
Yun said that the threat posed by North Korea is "more grave than ever" and that it is "nearing the final stage of nuclear weaponization."
The North has conducted two nuclear tests this year and 23 missile tests, deepening concern that is moving closer to having a nuclear-tipped missile that could reach the American mainland.