South Korea's spy agency on Friday expressed worries about the faster-than-expected progress in the technology of the Democratic People's Republic of Korea (DPRK)'s nuclear weapons miniaturization.

Chief of the National Intelligence Service (NIS) briefed South Korean lawmakers of the parliamentary intelligence committee on what the DPRK claimed was a successful "nuclear warhead explosion" test.

Two members of the ruling Saenuri Party and the main opposition Minjoo Party, who attended the briefing, told reporters that the NIS head expressed concerns about the country's significantly faster-than-expected speed at developing a miniaturized nuclear warhead.

Lee Byung-ho, the spy agency chief, said the DPRK's aim is to develop a nuclear warhead small enough to fit on even a short-range Scud missile.

Even if a warhead is placed on a ballistic missile, the weaponization is a different story, Lee noted, forecasting the weaponziation will not be achieved within one to two years.

Lee, however, said worries increase about faster-than-expected weaponization of the DPRK's nuclear program.

Touching on worries about additional nuclear tests, Lee said it would be no surprise to see Pyongyang conduct another test at any time, given the pace of progress of the fifth test.

Seoul's military estimates the latest test produced the biggest-ever explosive yield of 10 kilotons, stronger than 6 kilotons recorded in the January test in which Pyongyang claimed it detonated its first hydrogen bomb.

Lee said the fifth test was not a detonation of H-bomb, but the NIS chief noted that it cannot be regarded as a failure given the strength of the explosion.

The time gap between the fourth and fifth nuclear tests was only eight months, marking the shortest yet as Pyongyang tested atomic devices every three years or so in previous detonations. The first was staged in October 2006, followed by the second in May 2009 and the third in February 2013.

Kim Hwang-rok, head of the defense ministry's defense intelligence headquarters, said the military spotted signs of the DPRK's completion to prepare for the fifth nuclear test two to three months ago.

Kim said it would take two to three days to make a final confirmation over the fifth test after analyzing figures from gauging devices. The period can be extended to as late as seven days, he added.