North Korea Covers Tunnel at a Nuclear Site

SEOUL, South Korea — North Korea has put a cover over the entrance of a tunnel at its main underground nuclear test site to foil American intelligence efforts to determine whether a detonation there is imminent, a South Korean military official and media reported on Friday.

The news comes a day after a South Korean general said “brisk” activity had been spotted at the site. North Korea has said it will conduct a third nuclear test to retaliate against the United Nations Security Council’s unanimous decision last month to respond to a rocket test by tightening sanctions on the country. The North’s media cited the country’s top leader, Kim Jong-un, as ordering his military and government last week to take “high-profile” measures, suggesting that the test might happen soon despite international warnings against it.

In recent months, American and South Korean officials have detected new tunneling activities and what appeared to be other efforts to prepare for another underground test at the site, Punggye-ri in northeastern North Korea, where the country conducted tests in 2006 and 2009.

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 and anger the United States and other western countries alarmed at the North’s recent advances in its arms programs. 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. American and South Korean officials have been scrutinizing daily updates from satellite imagery of the Punggye-ri site, which features three tunnels dug into a 7,380-foot-tall mountain.

Still, predicting when a test might happen has been difficult because the satellites cannot observe what is going on underground. So the officials had been zeroing in on the entrance of the newest of the tunnels, where a test is considered most likely.

The 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 the tunnel was covered recently. South Korean news media, including the national Yonhap news agency, also cited military sources on Friday in reporting the recent covering.

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

In 2010, a South Korean warship exploded and sank, killing 46 sailors. The United States and South Korea blamed a torpedo attack by a North Korean submarine, despite denials from the North.

American nuclear submarines have occasionally visited South Korean naval ports, and North Korea has often cited such port calls in justifying its efforts to build what it calls a “nuclear deterrence.”