During a phone call Tuesday, Mattis and South Korean Defense Minister Song Young-moo discussed North Korea's sixth and most powerful nuclear test conducted over the weekend.
"Secretary Mattis assured Minister Song that the United States remains ironclad in its commitment to the defense of the ROK,"
Pentagon chief spokesperson Dana White said in a statement, using the acronym of South Korea's official name, the Republic of Korea.
"He further emphasized that any threat to the United States, its territories, or its allies will be met with a massive, effective, and overwhelming military response."
Mattis made similar remarks following a meeting with President Donald Trump on Sunday.
The defense chiefs also discussed ways to improve bilateral, trilateral and regional defense cooperation for the peace and stability of Northeast Asia and the broader Asia-Pacific region, White said.
In a separate statement, South Korea's defense ministry earlier said Song and Mattis agreed to bolster the combined defense posture and deter North Korean provocations by strengthening the regular deployment of U.S. strategic military assets and joint military exercises on and around the Korean Peninsula.
They also agreed that the international community should fully implement current sanctions on North Korea and adopt new punitive measures, it said. (Yonhap)