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North Korea has said it would conduct a third nuclear test to retaliate against the United Nations Security Council's unanimous decision last month to tighten sanctions on the country. Its media cited its top leader, Kim Jong-un, as ordering his military and government last week to take "high-profile" measures, indicating that a third nuclear test might happen soon.

In recent months, U.S. and South Korean officials have detected new tunneling activities and what appeared to be other efforts to prepare for a third underground nuclear test at Punggye-ri in northeastern North Korea, where the country conducted an underground nuclear test in 2006 and again in 2009. North Korea can now conduct a nuclear test any time once its leadership in Pyongyang makes up its mind, officials here said.

The North Korean threats have kept officials and analysts in the region on tenterhooks as any test is likely to aggravate tensions on the Korean Peninsula. Earthquake monitoring stations and military planes are on standby to detect seismic tremors and measure increased radiation in the air in case of a detonation in the North. U.S. and South Korean officials were scrutinizing daily updates from satellite imagery of the Punggye-ri site, which features three tunnels dug into a 7,380-foot-tall mountain and multiple support buildings.

Still, predicting when a test might happen has been difficult because the satellites cannot observe what was going on under the tunnels. So U.S. and South Korean officials have been particularly zeroing in on the entrance of the newest of the three tunnels, where a test was most likely; sealing the entrance would be a clearest sign of an imminent test.

Lately, they have faced a further complication.

A South Korean military official, speaking on condition of anonymity because he was not authorized to talk to the media on the record, said on Friday that North Korea has recently put a large cover at the entrance of the tunnel in an apparent attempt to block U.S. spy satellites from monitoring what was happening there. South Korean news media, including the national Yonhap news agency, also cited military sources on Friday in reporting on such a cover.

The official's remarks elaborated on a comment that Army Gen. Jung Seung-jo, the chairman of the South Korean military's Joint Chiefs of Staff, made a day earlier.

"We have detected brisk activities at Punggey-ri," General Jung told reporters on Thursday. "We are watching closely whether this is for a nuclear test or is just a camouflage."

His comment was made the same day as President Lee Myung-bak called a meeting of security-related officials to warn that North Korea would face much tougher sanctions if it pushed ahead with a nuclear test in defiance of the international community's warnings. But General Jung's comment was embargoed until Friday.

Days before North Korea launched its rocket in December, North Korea put a cover over its rocket launching pad, making it difficult for U.S. and South Korean intelligence officials to monitor the area. It then told the rest of the world that it was having technical problems.

A day before North Korea launched its rocket on Dec. 12 and successfully put a satellite into orbit, some officials and analysts in Seoul said that a long delay was likely, some of them even suggesting that the rocket had been removed from the pad for fixing. Many U.S. officials were also caught off-guard by the subsequent launching.

The Security Council adopted a resolution on Jan. 22 to punish North Korea for the rocket launching, which it considered a test of intercontinental ballistic missile technology. Earlier resolutions banned the country from such tests.

"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."

General Jung made the comment to domestic reporters touring a nuclear-powered U.S. submarine on Thursday.

Also on Thursday in Washington, speaking at his confirmation hearing before the Senate Armed Services Committee, Chuck Hagel, President Obama's nominee for defense secretary, said, "North Korea is beyond a threat. It's a real nuclear power and quite unpredictable."

As the developments were unfolding, the U.S.S. San Francisco, a Los Angeles-class nuclear submarine, was docked at the Jinhae naval base on the southern coast of South Korea ahead of a joint U.S.-South Korean submarine exercise slated for next week. Gen. Jung said the drill was not timed to North Korean moves for a possible nuclear test, but added that South Korea and its American ally were guarding against possible North Korean provocations involving submarines.

In 2010, a South Korean warship exploded and sunk, killing 46 sailors. The United States and South Korea blamed it on a torpedo attack by a North Korean submarine, despite Pyongayng's denials.

U.S. nuclear submarines have occasionally visited South Korean naval ports, and North Korea has often cited such port calls in justifying its attempt to build what it calls a "nuclear deterrence."

North Korea said its upcoming atomic explosion will be a "high-level" test. A new test can help analysts determine whether North Korea was making advances in its bomb-building technology. The analysts say North Korea may this time detonate a device made from highly enriched uranium, which would show that the country has acquired a second source for manufacturing bombs in addition to plutonium.

North Korea watchers in South Korea are speculating on various dates for a possible nuclear test. Some predict that it would happen before the Feb. 16 birthday of Kim Jong-il, the late North Korean leader and father of Kim Jong-un, who has inherited leadership after the senior Kim's death in December 2011.