TOKYO, May 25 -- North Korea exploded a nuclear device Monday morning, which is its second underground test in three years and is part of a pattern of escalating belligerence this year that has included a missile launch and withdrawal from all nuclear negotiations.

The communist state's official Korean Central News Agency described the test as "successful." It occurred at 9:45 a.m. in a northeast part of the country near where the first test was conducted in October 2006, according to reports from the South Korean government.

"The republic has conducted another underground nuclear testing successfully in order to strengthen our defensive nuclear deterrence," the North Korean news agency said.

The explosion produced a 4.7-magnitude tremor, according to the U.S. Geological Survey. The previous nuclear test registered 3.58 on the Richter scale.

"We are aware of the reports of a nuclear test by North Korea," a State Department official said late Sunday, speaking on condition of anonymity because the test was not yet confirmed. "We are consulting with our allies. Once we have established the facts, we will have more to say."

The test challenges the Obama administration, which came into office saying it was eager to make progress on the nuclear impasse with North Korea.