The U.S. government is looking into reports of unusual seismic activity around North Korea's nuclear test site, an official said Monday.

The White House is expected to release Washington's formal response when the 5.1 magnitude tremor is confirmed to be from a nuclear test, added the U.S. government official who spoke on the condition of anonymity.

"We're looking into it. I think you should keep an eye out for a White House statement," the official said.

A White House official, however, added, "We have nothing right now."

The U.S. Geological Survey (USGS) said earlier it had detected magnitude 4.9 earthquakes in a northeastern region of North Korea where the communist nation detonated nuclear bombs in 2006 and 2009.

The U.N. Security Council is scheduled to hold an emergency session on Tuesday morning to discuss the issue in New York.

North Korea's apparent nuclear test came on the eve of U.S. President Barack Obama's State of the Union speech, the first of his second term.

Obama plans to re-emphasize his nuclear-free world vision in the speech at Capitol Hill, according to administration officials.

Pyongyang's move is seen as a blow to Obama's initiative for nonproliferation and arms control.

"As North Korea is almost certain to have carried out another nuclear test, chances are high that President Obama will talk about the North Korea issue in his State of the Union address," a diplomatic observer said.

For many, the possibility of North Korea's third nuclear test came as no surprise since it has publicly threatened to do so amid activities at its nuclear test area.

One key question is whether North Korea used uranium. In its two previous experiments, it detonated plutonium-made bombs.

A uranium-based nuclear device poses a more serious threat to global security because it is very hard to trace.

U.S. officials have delivered a consistent message that Washington would cope with a nuclear test in a stern and resolute manner.

"We don't take anything off the table. We never do, " State Department spokeswoman Victoria Nuland said last week during a press briefing.

The U.S. and its allies will likely focus first on hammering out a unified response in the U.N. Security Council, followed by their own punitive steps.

In a new resolution against North Korea for its December long-range rocket launch, the council expressed "its determination to take significant action in the event of a further DPRK launch or nuclear test."

If Pyongyang actually conducted a nuclear test, experts say, it would test not only Obama's patience but also the policy of China's leader-in-waiting Xi Jinping.

"We will soon learn the Chinese definition of 'serious action,'" said Gordon Flake, the executive director of the Maureen and Mike Mansfield Foundation.

Nicholas Eberstadt, a researcher at American Enterprise Institute, said North Korea's continued nuclear tests and ballistic rocket launches are not a surprise.

  "For North Korean leadership, doing so makes urgent sense -- for this is an engineering matter of the highest priority for that state," he said. "Pyongyang will continue to test nukes and missiles until it is confident it has perfected its capability for targeting the U.S. homeland." (Yonhap News)