By Cole Hill ( | First Posted: Feb 01, 2013 03:37 PM EST

A screen shows a rocket being launched from a launch pad at the West Sea Satellite Launch Site, at North Korea's satellite control centre in Cholsan county, North Pyongan province. (Photo : Reuters)

North Korea has made brisk progress in preparing its main underground nuclear test site, but has put a cover over the entrance to the tunnel to thwart attempts to discern whether a detonation might be imminent, South Korean officials and media reported on Friday.

North Korea denounced the U.S. as its "sworn enemy" and announced more nuclear tests earlier in January in retaliation for the United Nations Security Council's unanimous decision to tighten sanctions in the insular nation. North Korean leader Kim Jong-un was said to have ordered his military and government to take "high-profile" measures last week, according to the country's media, the New York Times reported, that indicated a third nuclear test could happen soon.

American and South Korean officials have detected new tunneling activities in recent months. They have also discovered what appeared to be other preparation efforts for a third underground nuclear test at Punggye-ri in northeastern North Korea, the same region where the nation made an underground nuclear test in 2006 and 2009. North Korea now has the ability to conduct a nuclear test any time its leadership decides, officials said.

South Korea is on stand by with its earthquake monitoring stations and military planes, ready to detect seismic tremors and measure increased radiation in the air in case of a North Korean nuclear test. Satellite imagery from the Punggye-ri site, which features three tunnels dug into a 7,380-foot-tall mountain and several support structures, is being scrutinized daily by American and South Korean officials.

With the nuclear test sites underground, satellites cannot fully observe the reality of the situation, drawing the attention of American and South Korean officials to the entrance of the newest of the three tunnels in particular, where a test is most likely. A sealed entrance to the tunnel would be the most obvious sign of an impending test, according to the New York Times.

Speaking on the condition of anonymity because he wasn't authorized to talk to the media on the record, a South Korean military official said Friday that North Korea recently had put a large cover over the entrance of the tunnel, apparently in an effort to block American spy satellites from monitoring activity at the site. According to the New York Times, South Korean news media also cited military sources that claimed such a cover had been placed over the tunnel's entrance.

South Korean officials say they are watching closely to determine if the cover is camouflage or a sign of an imminent nuclear test.

The United Nations Security Council adopted its resolution to punish North Korea for its rocket launching Jan. 22.

"The North Koreans engaged in deceptive moves before they launched a long-range missile, and this time too, there is a limit in our monitoring because things are taking place underground," General Jung said. "We stay vigilant 24 hours a day because a nuclear test can happen any time."

North Korea experts are keeping a constantly watchful eye on the country, speculating on possible dates for a nuclear test. Some think a test could happen before Feb. 16 - the birthday of Kim Jong-il, the late North Korean leader, and father of current leader Kim Jong-un.

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