US Secretary of Defense James Mattis reaffirmed America's commitment to its mutual defence treaty with Japan. Earlier Mattis said N Korea would suffer an "effective and overwhelming" response if Pyongyang chose to use nuclear weapons.
US President Donald Trump's defence secretary on Friday reaffirmed America's commitment to its mutual defence treaty with Japan.
James Mattis made the pledge during a meeting with Japan's Prime Minister Shinzo Abe in Tokyo.
"I want to make certain that Article 5 of our mutual defence treaty is understood to be as real to us today as it was a year ago, five years ago – and as it will be a year, and 10 years, from now," Mattis said.
Article 5 obliges the United States to defend territories under Japanese administrative control.
Mattis is on his first trip since taking over the Pentagon, following the election of Trump as president.
TRT World's Mayu Yoshida in Tokyo has more on Mattis's visit.
US sidesteps China on visit to northeast Asia allies
Trump singled out South Korea and Japan on the campaign trail, suggesting they were benefiting from the US security umbrella without sharing enough of the costs.
Trump also jolted the region when he pulled the US out of an Asia-Pacific trade deal that Japan had championed.
Mattis's visit to Japan comes just a week ahead of a trip to Washington by Prime Minister Abe, and talks with Trump on February 10.
Earlier on Friday, Mattis warned North Korea of an "effective and overwhelming" response if Pyongyang chose to use nuclear weapons.
Mattis was speaking at South Korea's defence ministry on the last day of his two-day visit to that country.
"Any attack on the United States, or our allies, will be defeated, and any use of nuclear weapons would be met with a response that would be effective and overwhelming," Mattis said.
During the visit, the US and South Korea agreed, "to deploy and operate Terminal High Altitude Area Defense (THAAD) system within this year as planned, which is a defence system solely against North Korea's missile threat."
North Korea is thought to be readying to test a new ballistic missile, in what could be an early challenge for Trump's administration.
North Korea regularly threatens to destroy South Korea and its main ally, the United States. It conducted more than 20 missile tests last year, as well as two nuclear tests, in defiance of UN resolutions and sanctions.
Absent from Mattis's agenda was a visit to Beijing, in many respects the regional power broker, and at odds with Japan and South Korea on a number of issues.
China objects to THAAD, saying it will destabilise the regional security balance.
Beijing also claims the Diaoyu islands, which are currently under Japanese administration, called the Senkakus by Tokyo.
Japan has been keen for assurances that Trump's administration would continue Washington's previous policy of committing to defend the disputed East China Sea islands.