North Korea's state-run Korean Central News Agency (KCNA) says the communist country has conducted another "productive" underground nuclear test.

KCNA said the test was part of the North's efforts to further strengthen its nuclear deterrent.

Pyongyang officials were reported as saying that today’s nuclear test was conducted safely and that the results show the North has improved its nuclear capabilities since its last test in October 2006.

South Korean intelligence agencies said a magnitude four-point-five earthquake was detected near the town of Punggye in Gilju County, North Hamgyeong Province, at 9:54 a.m. Monday.

The region, which is known to be one of the areas least susceptible to earthquakes in the North, has been suspected of housing a North Korean nuclear test site.

Quoting analysis compiled by the Korea Institute of Geoscience and Mineral Resources, a presidential official said the tremor was artificially created.

President Lee Myung-bak convened an emergency National Security Council meeting Monday afternoon to review the situation.

South Korea, U.S. and Japanese intelligence officials are in the process of analyzing the data and remain on heightened alert.