SEOUL, Oct. 27  -- South Korea's defense chief said on Monday that the Democratic People's Republic of Korea's (DPRK) technology to miniaturize nuclear weapons appears to have reached a significant level.

"It is estimated that (the DPRK's) miniaturization technology ( for nuclear weapons) has reached a significant level," Defense Minister Han Min-koo told lawmakers during a parliament hearing. " It is appropriate for the military to maintain readiness based on such assessment."

The DPRK carried out three nuclear tests, and a substantial amount of time has passed since the first nuclear test, Han said, indicating the DPRK is coming closer to mounting nuclear warheads on its ballistic missiles.

Miniaturizing nuclear warheads has been considered the core process in developing nuclear missiles for the DPRK. Pyongyang conducted three nuclear tests in October 2006, May 2009 and February 2013 and test-fired a number of long-range missiles.

Asked whether the DPRK is capable of building uranium-based atomic bombs, Han said "Yes," without elaborating in details.

The South Korean military and intelligence agency believe that the DPRK has been capable of producing as much as 40 kg of highly enriched uranium per year since late 2010.