WASHINGTON -- US Defense Secretary Jim Mattis on Thursday has reaffirmed his country's commitment to the defense of South Korea, saying his troops could "fight tonight" if the Asian ally were to come under attack.

Mattis was speaking to reporters en route from Vietnam to Hawaii near the end of a regional trip that also took him to Indonesia.

"The defense of South Korea is a treaty alliance with us," he said, according to an official transcript. He explained that the 1950-53 Korean War ended in an armistice, not a peace treaty, leaving military options in place to respond to North Korean aggression.

"And numerous times, North Korea has violated (the armistice) over the years, and out in the waters, and even against airliners -- civilian airliners, that sort of thing," he continued. "We could fight tonight, shoulder to shoulder with the Koreans -- or South Koreans, if they're attacked."

Mattis made clear that the military options are meant to back diplomatic efforts to denuclearize the North.

"They exist so that the diplomats speak from a position of authority, that they have to be listened to, because an attack on the Republic of Korea will be severely rebuffed if it's attempted," he said, using South Korea's official name.

Asked if the existence of those military options could affect North Korean leader Kim Jong-un's calculations, Mattis dodged a direct answer.

"You'd have to ask Kim Jong-un that," he said. "I've got modest expectations that I can predict Kim Jong-un."

The Pentagon chief is scheduled to meet with South Korean Defense Minister Song Young-moo in Hawaii Friday.

He hailed the bilateral relationship, saying the two sides have "very transparent communications" at "all levels, every day."

"It's very open communication, absolute trust between us," he said. "The tensions are the tensions of the issues, not of the between us." (Yonhap)