North Korea is ready to conduct another nuclear test, but no imminent sign has been detected at its main site in the northeastern tip, South Korea's defense ministrysaid Friday.

South Korea's vice defense minister Baek Seung-joo briefed lawmakers of the ruling Saenuri Party on the condition of the Punggye-ri test site in its northeastern region during a meeting of the special security committee on North Korea's nuclear weapons amid rising speculation over another test.

The southern tunnel of the underground test site can be used for another explosion at any time, but currently there are no imminent signs for such action, Baek was quoted as saying by participants.

The western tunnel, where the third explosion took place in February, is currently under construction, Baek said. Pyongyang is expected to acquire 6 km of weapons-grade plutonium by the end of next year if the Yongbyon nuclear reactor continues to operate in the current phase, he noted.

The communist state has resumed operations at its previously mothballed Yongbyon nuclear complex, which would allow the North to extract plutonium from spent fuel rods.

The nation's main nuclear reactor was shut down under an agreement reached at nuclear disarmament-for-aid talks in 2007. The assessment comes as experts raise suspicion that North Korea is preparing for another nuclear test that could be conduct any time soon for the purpose of gaining a strategic edge in resuming multilateral nuclear disarmament talks, which have been halt since 2008.

On Thursday, Victor Cha, a North Korean expert and former White House security official, said the North could conduct its fourth nuclear weapons test as early as late this year.

Recent satellite imagery showed that North Korea has made new tunnel entrances at its nuclear test site in a sign it is preparing to conduct more underground explosions there in the future, said 38 North, a website operated by the U.S.-Korea Institute at Johns Hopkins School of Advanced International Studies.

Seoul's defense ministry estimates that North Korea has made considerable progress in its nuclear development program in light of its third nuclear test. But many still doubt whether the impoverished state has mastered the technology necessary to produce a nuclear warhead that can be mounted on an inter-continental ballistic missile. (Yonhap news)