Mattis: Any threat by North Korea will be met by massive US military response

"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 at the White House with the chairman of the Joint Chiefs of Staff, Gen. Joseph Dunford - at his side.

Mattis spoke briefly, but took no questions from reporters, after he met with President Donald Trump to discuss North Korea's claim.

Meanwhile, the US government is studying new sanctions on North Korea after the latest nuclear test by the regime of Kim Jong-un, US Treasury Secretary Steven Mnuchin said Sunday.

"It's clear this behavior is completely unacceptable," Mnuchin told Fox News. "We've already started with sanctions against North Korea, but I am going to draft a sanctions package to send to the president for his strong consideration that anybody that wants to do trade or business with them would be prevented from doing trade or business with us."

Mnuchin said he had spoken with Trump and the latter also believes that North Korea's behavior is "completely unacceptable."

Trump on Sunday on Twitter condemned the latest nuclear test by North Korea, a country that he called "hostile" and "dangerous," and that - he emphasized - has become a source of "embarrassment" for China.

"North Korea has conducted a major Nuclear Test. Their words and actions continue to be very hostile and dangerous to the United States.......North Korea is a rogue nation which has become a great threat and embarrassment to China, which is trying to help but with little success," said Trump in a pair of tweets.

South Korea "is finding, as I have told them, that their talk of appeasement with North Korea will not work, they only understand one thing!" the president added in a subsequent tweet.

"The United States is considering, in addition to other options, stopping all trade with any country doing business with North Korea," Trump said in yet another Sunday morning Twitter post.

North Korea on Sunday tested its most powerful atomic weapon to date, a device that the regime said it could install onto an intercontinental ballistic missile, which - if confirmed - would be an important and dangerous declaration of its military capabilities.

This is the sixth North Korean nuclear test during the course of its frenetic weapons development program in recent years.

Kim Jong-un has also tested more than 10 ballistic missiles since the beginning of the year, including two intercontinental ballistic missiles that could potentially reach other countries.