North Korea appears to have achieved "a significant level" of technology to build miniaturized nuclear weapons that could be mounted on its long-range ballistic missiles, South Korea's defense chief said Monday.

"North Korea is assumed to have a significant level in its miniaturization technology. It is appropriate for the military to maintain readiness based upon the assessment," Han Min-koo said during the regular parliamentary audit into the ministry.

For decades, Pyongyang has been working to build nuclear-tipped missiles. It has carried out three nuclear tests so far and test-fired a number of long-range rockets. Miniaturizing nuclear warheads has been considered one of the last-remaining hurdles to its development of nuclear missiles.

Han's comments echoed the remarks last week by Gen. Curtis Scaparrotti, who leads the United States Forces Korea, that the North has "the capability to have miniaturized the device at this point" though he did not have knowledge of whether it has actually built such a weapon.

Asked if the communist country is capable of building a uranium-based nuclear weapon, Han said, "Yes," without further elaboration.

The intelligence authorities here believe that the unruly communist nation has been able to produce up to 40 kilograms of highly enriched uranium per year since 2010, enough for up to two atomic weapons.

Vowing efforts to effectively deter the North, Han stressed the government's stance that it will never acknowledge its communist neighbor as a nuclear state.

North Korea's evolving nuclear and missile threats caused South Korea and the U.S. to delay Seoul's taking back the wartime operational control (OPCON) of its troops from Washington.

The decision to push back the date until the mid-2020s when South Korea is expected to acquire key assets to counter such threats was made last week during the annual defense chiefs' talks of the Security Consultative Meeting (SCM) in Washington. 

The delay, second of its kind since 2006 when the two sides first agreed upon the handover, has been a source of heated debate at home, with critics bashing the government for "giving up its sovereignty in defending its own territory."

"The OPCON transfer is not a matter of sovereignty ... Under current circumstances on the Korean Peninsula, we should be prepared for war by maintaining the maximum level of effective joint defense posture," the minister said.

Asked if he "does not feel shameful" for failing to exercise the command after 60 years of the 1950-53 Korean War, Han said, "No. The delay does not stem from the lack of confidence. We just need extra time to guarantee the additional capabilities to deter North Korea's aggression."

South Korea was scheduled to reassume wartime operational control in December 2015 after handing it over to the U.S. during the Korean War, but it asked for shelving the plan last year in the wake of the North's continued nuclear pursuit. (Yonhap)