Any North Korean nuclear attack against South Korea will be met with a “swift,” “overwhelming” and “decisive” response, a senior U.S. diplomat said Thursday, amid growing concerns over Pyongyang’s continued weapons tests and bellicose rhetoric.

Speaking at a forum, Bonnie Jenkins, the under secretary of state for arms control and international security, reiterated the U.S. policy stance while expressing “serious concern” over growing military cooperation between Pyongyang and Moscow.

“It is important to recall U.S. policy on this issue: Any nuclear attack by North Korea against the United States and its allies is unacceptable, and will result in the end of the Kim regime,” she said at the forum hosted by the Korea Society in New York.

“And any nuclear attack by the DPRK against the ROK will be met with a swift, overwhelming and decisive response,” she added. ROK and DPRK stand for the official names of South Korea and North Korea, the Republic of Korea and the Democratic People’s Republic of Korea, respec
tively.

Her remarks came amid lingering doubts over America’s “extended deterrence” commitment to using the full range of its military capabilities, including nuclear, to defend its ally.

The doubts persisted as Pyongyang has been doubling down on nuclear warheads and its delivery capabilities, which analysts warn could threaten the security of the U.S. mainland and thus keep American forces at bay in a crisis.

While stressing the U.S.’ “ironclad” defense commitment to South Korea, Jenkins pointed out Washington’s steady commitment to diplomacy with the North.

“Our efforts to maintain and strengthen deterrence must be paralleled by our continued charge towards diplomacy and dialogue to resolve difficult security challenges,” the ambassador said.

“As such, both the U.S. and the Republic of Korea remain steadfast in our pursuit of dialogue with the DPRK without preconditions and see diplomacy as the only viable means of achieving a lasting peace on the Korean Peninsula.”

On deepening cooperation between t
he North and Russia, Jenkins characterized it as a “very serious concern and serious threat,” noting that it is providing Russia with “more and more ways” to continue Moscow’s war in Ukraine.

“We are looking at all of our tools and figuring out how we can strengthen and adjust our tools to address what is a very concerning situation and a very dangerous relationship,” she said.