He says Tillerson was stepping beyond his administration role as the nation's top diplomat. President Trump has made revamping USA missile defense systems a top objective for the Pentagon since taking office.
In a process dubbed the "New York channel", Joseph Yun, the USA envoy for North Korea policy and the only American to meet with North Koreans, passes messages back and forth between the two nuclear-armed nations.
Trump's remarks have prompted a reaction in China, where the state-controlled Global Times newspaper said in an editorial Friday that if North Korea launched missiles at the USA, it should "stay neutral", sending a warning to Pyongyang that Beijing would not support the regime if it made a first move.
In a lengthy exchange with reporters at his Bedminster golf resort, he said past administrations had not done enough to take on North Korea and that it was time a president "stuck up for the country".
Speaking at an event in California on Thursday, Mattis said his mission and responsibility was to have military options ready "should they be needed" but stressed the United States effort is now focused on diplomacy.
Even as President Donald Trump says the U.S. military is "locked and loaded" to potentially unleash "fire and fury" on North Korea, the two feuding countries have been holding secretive, back-channel talks, The Associated Press reported Friday.
USA intelligence reports claiming that North Korea has miniaturized a nuclear bomb to fit on top of intercontinental ballistic missiles capable of reaching the United States have prompted Trump to threaten massive retaliation if Pyongyang attacks.
The Kim Jong-Un regime has said it now has the capability of striking the USA mainland after two successful intercontinental ballistic missile (ICBM) tests last month.
He points out the U.N. Security Council unanimously voted last week to characterize North Korea's statements as a "threat to the world's community".
Responding to a North Korea's threat to fire four missiles in the waters off the U.S. Pacific territory of Guam, home to U.S. military bases, Trump vowed a tough response.
"The UK stands shoulder to shoulder with the U.S.in tackling this threat and many others". We call on the relevant parties to be cautious with their words and actions, and contribute more toward easing tensions and enhancing mutual trust, ' Geng said in a statement.
"He would lash out militarily, at a minimum against South Korea", Admiral Stavridis said to NBC. "And I don't think we are".
And he points to another sign that the U.S.is not moving toward war with North Korea: There have been no efforts to evacuate the at least 150,000 USA citizens living in South Korea.