The comments from defense secretary Jim Mattis came after President Donald Trump called a meeting of his national security advisers to discuss an unexpectedly powerful nuclear test said to exceed in magnitude the atomic bombing of Hiroshima, Japan.
Trump had earlier denounced the test, saying the time for "appeasement" was over and threatening drastic economic sanctions, including "stopping all trade with any country doing business with North Korea."
Mattis went futher still, telling North Korean leader Kim Jong-Un: "Any threat to the United States or its territories, including Guam, or our allies will be met with a massive military response, a response both effective and overwhelming.
"We are not looking to the total annihilation of a country, namely North Korea. But as I said, we have many options to do so," he added.
US monitors measured a powerful 6.3-magnitude earthquake near the North's main testing site, with an aftershock possibly caused by the collapse of a tunnel at the site.
The resultant shock caused momentary panic in parts of China -- and Trump slammed it as "very hostile and dangerous to the United States."
South Korea launched a ballistic missile exercise in response to Pyongyang's provocative detonation of what it claimed was a miniaturised hydrogen bomb, state news agency Yonhap reported.
The South's military conducted a live-fire exercise simulating an attack on the North's nuclear site, hitting "designated targets in the East Sea", the report added, quoting the Joint Chiefs of Staff.
"The training came in response to the North's sixth nuclear test... and involved the country's Hyunmoo ballistic missile and the F-15K fighter jets," it said.
