Heightened activity at North Korea's nuclear test site may indicate that the reclusive country is getting ready to conduct another underground detonation in the near future, military officials here said Wednesday.

South Korea's military said it will remain on high alert as the North might launch another long-range missile or conduct a sixth nuclear test or both next month to celebrate the 68th anniversary of the founding of the ruling Worker's Party of Korea on Oct. 10. 

"The canopy erected to hide the entry way of No. 2 tunnel (where the fifth nuclear test was conducted) has not been removed.

Moreover, a giant canopy was set up over the entryway of the No. 3 tunnel where many expect a sixth nuclear test could take place," a military official told Yonhap News Agency. 

The North's fifth nuclear test was conducted at the No. 2 tunnel in the Punggye-ri area in North Hamgyeong Province. The canopy is not camouflaged, but does prevent accurate observation of the area it covers from the sky.

Both No. 2 and the No. 3 tunnels could be used at any time for another nuclear test in Punggye-ri, the site of all five nuclear explosions, the official said.

Pyongyang has carried out five nuclear tests in the past decade. Three of them took place under the Kim Jong-un regime since he took power from his deceased father Kim Jong-il in December 2011.

All nuclear device detonations were made at either the No. 1 or No. 2 tunnel at Punggye-ri.

The North most recent test conducted on Sept. 9 was the strongest to date, After the detonation Pyongyang claimed it had mastered the ability to mount a warhead on a ballistic missile.

Following up on this provocation, the North on Tuesday carried out a "successful" ground test of a new rocket engine that could further bolster its nuclear and missile capabilities.

After supervising the engine test, North Korean leader Kim Jong-un called on officials, scientists and technicians "to round off the preparations for launching the satellite as soon as possible," hinting an imminent missile launch. (Yonhap)