Defense Secretary Ash Carter on Wednesday spoke by phone with his South Korean counterpart in the wake of a purported North Korean hydrogen bomb test.
A Pentagon statement said Carter and South Korean Minister of National Defense Han Min-koo discussed "potential alliance responses to the recent apparent North Korean nuclear test."
"Secretary Carter and Minister Han agreed that any such test would be an unacceptable and irresponsible provocation and is both a flagrant violation of international law and a threat to the peace and stability of the Korean peninsula and the entire Asia-Pacific region," the statement said.
During the call, Carter reaffirmed the "ironclad" commitment of the United States to the defense of South Korea, the Pentagon said, adding that that commitment includes "all aspects of the United States's extended deterrence."
"Minister Han emphasized the strength of the U.S.-[South Korea] alliance and its vital role in assuring peace and stability on the Korean Peninsula and across the Asia-Pacific. Carter and Han agreed that North Korea's provocations should have consequences," the statement said.
The U.S. has almost 30,000 troops in South Korea enforcing the demilitarized zone between North and South Korea established following a ceasefire between the two sides after the Korean War.
Carter also received an update from Army Gen. Curtis Scaparrotti, the commander of U.S. Forces Korea.