"We are prepared, we're prepared militarily, we're prepared with our allies to respond militarily," Tillerson told reporters, speaking at the close of US-Japan security meetings with Defense Secretary James Mattis.
The top US diplomat's message was underscored by Mattis, who also made clear the US' willingness to use force if North Korea steps out of line.
"In close collaboration with our allies, there are strong military consequences if the DPRK initiates hostilities," he said.
Tillerson also addressed the terror attack in Barcelona
Thursday that left at least a dozen dead and more than 80 injured, vowing that the US stands ready to help Spain's law enforcement officials
"Terrorists around the world should know the United States and our allies are resolved to find you and bring you to justice," Tillerson said.
The Security Consultative Committee, or "2+2" meeting, with Japanese Foreign Minister Taro Kono and Defense Minister Itsunori Onodera focused heavily on the threat posed by North Korea's missile launches and nuclear ambitions, Tillerson said.
"As you can imagine, we spent a fair amount of time discussing North Korea," Tillerson told reporters in a press conference after the meeting. He said that Pyongyang's missile launches were "unacceptable provocations -- and they must stop immediately." The US and Japan were taking steps in the face of that threat, he added.
But in an interview published Wednesday night, Bannon told American Prospect
magazine that North Korea's ability to wreak devastation on South Korea with its conventional weapons means that "they got us" -- making the US unable to intervene militarily.
Tillerson declined to comment on Bannon's interview, but he said, "It's quite clear what our policy toward North Korea is."
The top US diplomat laid out the international "peaceful pressure" campaign he is leading against the Democratic People's Republic of Korea, or DPRK, but in response to a question from CNN's Elise Labott, he added that the diplomatic approach "has to be backed with military threat" if North Korea chooses to move forward with destabilizing actions.
President Donald Trump fueled speculation about US intent to strike Pyongyang after he said that the northeast Asian nation would be met with "fire and fury"
if it continued threatening the US or moved to strike a US ally or territory.
Tillerson elaborated on that point Thursday: "The President felt it was necessary to remind the regime of what the consequences for them would be if they chose to carry out those threats," he said.
In the event of a North Korean missile launch toward Japan, Guam, South Korea or other allies, the US would take "immediate specific actions to take it down," Mattis said.
US, Japan ramp up their military alliance
In the face of the DPRK threat, Tillerson announced a decision to accelerate implementation of a 2015 agreement to overhaul the US military relationship with Japan.
The agreement enables Japan to come to the defense of any regional ally under attack, which means Japanese missile defense systems could be deployed to intercept weapons launched toward the US -- and it will create a standing group of Japanese and US defense and foreign policy officials to communicate and coordinate.
The US and Japan will also start new cooperation on intelligence, surveillance and reconnaissance, training and development, and the joint or shared use of facilities, among other things.
The broader aim of the peaceful pressure campaign is to increasingly isolate North Korea if it continues its current approach, Tillerson said, and for its leaders to see their situation as "bleak" and getting "bleaker."
Tillerson said the campaign differs from previous US approaches to North Korea because of its "level of international unity," the "level of cooperation from China and others" and the "intensity" of the campaign.
China's increasingly aggressive stance in Asia is another factor in the closer US-Japan relationship. Tillerson said he and Mattis had discussed tensions in the South China Sea, the subject of competing territorial claims between China and smaller countries in the region.
He warned that the US opposes "any unilateral action that seeks to undermine Japan's administration of the Senkaku Islands," which China also claims, and calls the Diaoyu Islands.
Tillerson warned that the US will honor treaty obligations to defend Japan "without reservation," and would be working to improve military alliances with other countries in the region, including South Korea, India and Australia.