SEOUL - The US Geological Survey said Tuesday it had detected a magnitude 4.9 earthquake in North Korea, but neither Pyongyang nor Seoul confirmed whether North Korea had conducted its widely anticipated third nuclear test.

The South Korean Defense Ministry said it was trying to determine whether North Korea had conducted a nuclear test. Nuclear blasts can create tremors but they are distinct from those caused by natural earthquakes.

North Korea's powerful politburo vowed to continue firing "powerful long-range rockets," but a statement Tuesday made no mention of Pyongyang's promise to conduct a nuclear test.

The United States and its allies have been on edge since North Korea said last month it will conduct its third nuclear test to protest toughened sanctions over a December rocket launch that the UN called a cover for a banned missile test.

North Korea's powerful National Defense Commission said January 23 that the United States was its prime target for a nuclear test and long-range rocket launches. North Korea accuses Washington of leading the push to punish Pyongyang for its December rocket launch.

Last October, a spokesman from the commission told state media that the country had built a missile capable of striking the United States, but did not provide further details. A missile featured in an April 2012 military parade appeared to be an intercontinental ballistic missile, but its authenticity has not been verified by foreign experts.