- North Korea said it has a hydrogen bomb designed for an ICBM
- A magnitude 6.3 detonation represented the North's most powerful yet
- U.S. Defense Secretary Jim Mattis warned of "a massive military response, a response both effective and overwhelming."
North Korea on Sunday conducted its sixth and most powerful nuclear test, which it said was of an advanced hydrogen bomb for a long-range missile, prompting the threat of a "massive" military response from the United States if it or its allies are threatened.
Speaking outside the White House after meeting with President Donald Trump and his national security team, U.S. Defense Secretary Jim Mattis said Trump asked to be briefed on all available military options.
"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," Mattis said.
It's not clear what Mattis means by "threat." North Korea regularly menaces the United States and its allies — it launched a missile over Japan last week and has verbally threatened South Korea, Australia and Guam in recent months.
"We are not looking to the total annihilation of a country, namely North Korea," Mattis said with Joseph Dunford, the chairman of the Joint Chiefs of Staff, at his side. "But as I said, we have many options to do so."
Trump earlier in the day refused to rule out military action and threatened to cut off trade with any country doing business with Pyongyang.
Asked while leaving a church service whether the United States would attack North Korea, Trump replied: "We'll see."
The United Nations Security Council is scheduled to meet on Monday to discuss the nuclear test. Mattis said the members of the council "remain unanimous in their commitment to denuclearization of the Korean Peninsula."
Last week, Trump said the time for talking was over, although Mattis later contradicted him, saying the United States had not exhausted all diplomatic options.
There was no independent confirmation that the detonation was a hydrogen bomb rather than a less powerful atomic device, but Japanese Chief Cabinet Secretary Yoshihide Suga said Tokyo
could not rule out such a possibility.
Experts who studied the impact of the earthquake, which the U.S. Geological Survey measured at magnitude 6.3, said there was enough strong evidence to suggest the reclusive state had either developed a hydrogen bomb or was getting very close.
—CNBC contributed to this report.