The U.S. has spotted a tarpaulin over a tunnel entrance at North Korea’s nuclear test site, a news report said Tuesday, adding to concerns that Pyongyang is preparing a fourth nuclear weapons test.

The tarp was spotted by a spy satellite late last week at the nuclear site in the northeastern town of Punggye, CNN quoted a senior Washington official as saying. A similar cover was used ahead of the North’s third detonation in February 2013, in an apparent attempt to hide the atomic device during its delivery to the site.

“If they put the tarp up, that could be a sign that they’re nearing that final stage, closing off the tunnel entrance, and that would be a huge concern to the U.S.,” the broadcaster said.

Yet it remains unclear if the tunnel entrance has been filled in, Seoul officials said, which they said would be a clearer signal of an imminent underground explosion.

Concerns have been growing since Pyongyang threatened a “new form of nuclear test” in March in protest against the U.N. Security Council’s condemnation of its test-firing of ballistic missiles.

President Park Geun-hye has said the Kim Jong-un regime is “fully ready” to detonate a device “at any time.” The South Korean military has also perceived an increase in vehicle and personnel movement, as well as a screen to cover a tunnel. It is not clear whether this is the same tunnel as the one cited in the recent report.

A recent series of analyses of satellite photos by the U.S.-Korea Institute at Johns Hopkins University suggested that recent excavation activities at the Punggye site may indicate that the North is building a tunnel complex to conduct multiple tests or explosions on a much more regular basis.

Defense Ministry spokesman Kim Min-seok said that whether the test will transpire “depends on North Korea’s political decision,” refusing to elaborate on intelligence matters.

Seoul has been ramping up diplomatic efforts with Washington, Beijing and other partners to dissuade its northern neighbor from another atomic provocation.

In New York, Foreign Minister Yun Byung-se said that stopping North Korea from carrying out another atomic weapons test and beefing up its nuclear and delivery capabilities were the “most pressing issues at hand.”

“Should North Korea, which is the sole nation to press ahead with a nuclear test in the 21st century, end up carrying one out again, it will have to pay the highest price, one that it has never experienced,” he said in a speech at the International Peace Institute.

During their talks Monday, Yun and European Union foreign affairs chief Catherine Ashton also stressed the need for the international community to respond “strongly” in the event of the nuclear blast.

By Shin Hyon-hee (heeshin@heraldcorp.com)