The United States, meanwhile, staged another show of force meant to deter any North Korean aggression by flying two B-1B supersonic bombers Tuesday night from an air base in the US territory of Guam to the South for drills with South Korean fighter jets.
It was part of a "regular deployment training" aimed at enhancing the capability of implementing the "extended deterrence" against the North, the JCS said.
Rhee, who belongs to the ruling Democratic Party and sits on the Defense Committee, told CNN on Tuesday that he received information about the alleged hacking from the Defense Ministry.
The U.S. bombers took off from the Andersen Air Force base in Guam, the U.S. Pacific territory that North Korea singled out as a possible target for a missile strike over the summer, as President Trump and North Korean leader Kim Jong Un engaged in bombastic rhetoric and brinkmanship.
Democratic Party representative Rhee Cheol-hee said in radio appearances on Wednesday that 235 gigabytes of military documents were taken from the Defense Integrated Data Center in September previous year, citing information from unnamed South Korean defence officials.
This year, North Korea tested an intercontinental ballistic missile that could potentially reach the USA mainland, and the US intelligence community assesses it has the technology to miniaturize a nuclear warhead atop it.
The data, said to be 235 gigabytes of military documents, was stolen from the Defense Integrated Data Center in September 2016.
"Flying and training at night with our allies in a safe, effective manner is an important capability shared between the US, Japan and the Republic of Korea and hones the tactical prowess of each nations' aviators", Major Patrick Applegate said in a statement. "What I am going to tell you is that the [South Korea]-US alliance, that bilateral entity, is there to deal with those types of situation and safeguard against them".
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"If the North Koreans in fact accessed the US/South Korean defense plans, this is a treasure trove of information and presents a real danger", said CNN military analyst and retired Lt. Col. Rick Francona.
"Our country has been unsuccessfully dealing with North Korea for 25 years, giving billions of dollars & getting nothing".
Despite the global sanctions, North Korean leader Kim Jong-un continues to develop nuclear weapons and advance his nation's ballistic missile program.
In a pair of tweets sent Saturday afternoon, Trump said past agreements with North Korea have all been violated.
The North Korean party anniversary passed without any missile launches or nuclear tests, but officials are wary the North could engage in more provocations at any time, especially with China to begin its all-important Communist Party Congress on October 18.
"Ladies and gentlemen, it is right now a diplomatically-led, economically sanction-buttressed effort to try to turn North Korea off this path", he said.
The mission was the latest show of force to North Korea after its long-range intercontinental ballistic missile (ICBM) tests and hydrogen bomb test that have ratcheted tensions with the United States.