In a statement, Mattis said Kim should "take heed" of the "fire and fury" warnings from President Donald Trump and cease his threats and provocations. "Hopefully we will never have to use this power, but there will never be a time that we are not the most powerful nation in the world!"
North Korea carried out two nuclear bomb tests previous year and two intercontinental ballistic missile tests in July.
"I want to reassure the people of Guam that now there is no threat to our island or the Marianas", he said.
"The situation is bad now, and it's going to get worse in August; it's going to get much more risky in August", Joel Wit, a senior fellow at US-Korea Institute and former State Department official, said in a Aug. 2 conference call with reporters organized by 38 North, a website for analysis of Korean defense issues.
US Secretary of State Rex Tillerson sought to play down the rhetoric.
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On Wednesday Trump tweeted the U.S. nuclear arsenal was "stronger and more powerful than ever before" but he hoped it never needed to be used. A U.S strike against North Korea would likely result in a retaliatory strike against American ally South Korea, where thousands of American troops have been stationed since the end of the Korean War. "They will be met with fire and fury like the world has never seen", Trump said.
North Korea will develop a plan by mid-August to launch four intermediate range missiles at the U.S. territory of Guam before presenting it to leader Kim Jong-un who will make a decision on whether to proceed, the North's state media said on Thursday. He said he had been in touch with the White House and there was no change in the threat level.
U.S. military officials played down the potential for military conflict.
Mattis said the United States and its allies have the capabilities and commitment to defend themselves.
The army will complete its plans in mid-August, ready for North Korean leader Kim Jong Un's order, KCNA said on Wednesday, citing General Kim Rak Gyom, commander of the Strategic Force of the Korean People's Army.