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.