PRESIDENT DONALD TRUMP: And if he utters one threat in the form of an overt threat, which, by the way, he has been uttering for years and his family has been uttering for years - or if he does anything with respect to Guam or any place else that's an American territory or an American ally, he will truly regret it.
And while Mattis warned on Wednesday that North Korea would be "grossly overmatched" by the US and its allies if a military conflict were to break out, some senior administration officials have provided a counterpoint to the President's more inflammatory rhetoric.
We are united in calling on the North Korean regime to halt its nuclear program.
Secretary Mattis, the former general whose nickname is Mad Dog, sounded cautious this week when he warned war with North Korea would be catastrophic.
Trump earlier showed no contrition about his remark on Wednesday that aggression by North Korea would be met with "fire and fury like the world has never seen".
Trump said North Korean leader Kim Jong Un was not going to get away with his "horrific" comments and disrespecting America.
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The president has a very good idea of the people that are undermining his agenda that are serving their own interests. I think at the end of the day; the president knows who the leakers are.
The State Department is holding a briefing a day after it was revealed North Korea has miniaturized nuclear warheads for use in ballistic missiles, leading to an exchange of threats between the US and the Kim Jong Un regime. And despite Trump's public bluster, the US has reportedly been carrying on quiet, back-channel negotiations with North Korea through that country's United Nations representatives.
The Northern Viper drills are one of the scheduled exercises that Japan's Self Defence Forces conducts regularly with their US counterparts and are not a response to the latest tensions.
The North Korea threat has been a foreign policy priority for the Trump administration since it took office in January, but it has taken center stage since Pyongyang test fired two intercontinental ballistic missiles last month.
South Korea's military says North Korea will face a "stern and strong" response from Washington and Seoul if it acts on threats to fire missiles near Guam.
South Korean and US troops are also gearing up for an annual joint drill from August 21, called the Ulchi Freedom Guardian, in which up to 30,000 USA troops will take part.
The United States and South Korea remain technically still at war with North Korea after the 1950-53 Korean conflict ended with a truce, not a peace treaty.