North Korea has stepped up activity at its nuclear site, Seoul’s Defense Ministry said Tuesday, in a fresh sign that the communist country may push ahead with a fourth atomic test in breach of international resolutions.

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

Though no signs were spotted of an impending underground explosion, the ministry said it set up an “integrated crisis management” task force Monday to prepare for any possibilities.

“We’re detecting lots of activity in North Korea’s nuclear site in the (northeast) town of Punggye,” ministry spokesperson Kim Min-seok told a news briefing.

“North Korea could suddenly carry out a nuclear test in the short term.”

But he suggested that the signs may mark another attempt by the Kim Jong-un regime to “deceive” the outside world, rather than actual preparations.

Other officials and analysts also raised the possibility that the increased activities are meant to be a flexing of military muscles in the run-up to U.S. President Barack Obama’s Asia tour this week. He is due to arrive in Seoul on Friday.

The South Korean military is believed to have perceived an increase in vehicle and personnel movement, as well as a screen to cover a tunnel.

This followed an analysis of satellite imagery last month by the U.S.-Korea Institute at Johns Hopkins University that concluded 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.”

Kim also mentioned that North Korean officials have spoken of a “next step that is unimaginable to enemies,” a “big event before April 30,” and “one big shot,” citing intelligence.

“North Korea is currently at the stage where it is capable of conducting a nuclear test unexpectedly if it decides so,” he added.

As tension escalates, Seoul has been ratcheting up diplomatic efforts with Washington, Beijing and other partners to dissuade its wayward neighbor from another atomic blast. Foreign Minister Yun Byung-se has warned Pyongyang of “unimaginable consequences.”

The chief nuclear negotiators of South Korea, the U.S. and Japan gathered in Washington early this month, sending a strong warning to North Korea against a fourth nuclear test and vowed to step up cooperation to preclude further provocations.

Hwang Joon-kook, Seoul’s new special representative for Korean peninsular peace and security affairs and top envoy to the six-party talks, also met with his Chinese counterpart Wu Dawei, China’s special representative for Korean affairs, in Beijing. Wu then traveled to the U.S. for consultations with Special Representative for North Korea Policy Glyn Davies last week.

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