SEOUL  -- The Democratic People's Republic of Korea (DPRK) actually completed all preparations needed for another nuclear test, though no immediate signs were detected yet, Seoul's Defense Ministry said Tuesday.

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The Republic of Korea's (ROK) Defense Ministry spokesman Kim Min-seok told a routine press briefing that the DPRK has been ready to conduct its fourth nuclear test on short notice from the leadership by building horizontal tunnels at the underground test site in its northeastern region.

The ROK's Defense Minister Kim Kwan-jin said in February that preparations for Pyongyang's another nuclear test were already completed, noting that Punggye-ri test site in the northeastern DPRK where its third test was conducted in February last year.

The top military official said nuclear tests and long-range missile launches could be carried out at any time upon the decision of the DPRK leadership, noting nuclear tests and missile launches were correlated as seen in the past cases.

The spokesman said that if minor equipment is moved to the tunnels, another nuclear test can actually be staged, but he noted such movement had yet to be detected.

The spokesman's comments came amid mounting concerns among the ROK's policymakers that the DPRK may carry out its fourth nuclear test as Pyongyang threatened a "new form" of such test.

On Sunday, the DPRK's Foreign Ministry said that it would "not rule out a new form of nuclear test to bolster up its nuclear deterrence," accusing the United States of "acting rashly" at the UN Security Council.

After Pyongyang's launch of two Rodong medium-range ballistic missiles last Wednesday, the UN Security Council held an extraordinary session, censuring the DPRK's violation of UN resolutions, banning the country from testing ballistic missile technology.

The ROK especially worried about the Rodong missile launches, which may signal another nuclear test.

The DPRK conducted its first nuclear test in October 2006, three months after firing the Rodong missiles. In May 2009, Pyongyang carried out its second nuclear test, just two months before firing off other Rodong missiles.

The third test was staged last February, just two months after Pyongyang launched the three-stage rocket called Unha-3, which Seoul claimed was a long-range missile.

The DPRK conducted its live-fire drill Monday afternoon, firing around 500 rounds of shells and projectiles with its artillery and multiple-rocket launchers from its shoreline gun positions in the DPRK's southwestern region near the Northern Limit Line (NLL), the disputed maritime border.

Some 100 of them fell in the South Korean waters 2-3 km south of the NLL, the scene of the inter-Korean naval skirmishes over the past years.

In November 2010, the DPRK shelled the Yeonpyeong Island, one of the five border islands near the NLL, killing four people and causing the ROK military's pounding of the North's shoreline gun positions.

Pyongyang has never recognized the NLL as it was unilaterally declared by the United Nations after the 1950-53 Korean War ended in armistice, not a peace treaty. The DPRK has claimed another line farther south.

Tensions escalated on the Korean Peninsula after Pyongyang fired a volley of short- and medium-range missiles in recent weeks to pretest against the joint annual war games between the ROK and the United States.

The "Key Resolve" command post exercise, which began on Feb. 24, ended on March 6, but the "Foal Eagle" field training exercise will last until April 18 despite the DPRK's earlier call for delay or cancellation of the drills, which Pyongyang has denounced as the rehearsal for a northward invasion.