The Democratic People's Republic of Korea (DPRK) said it has "successfully" conducted its third underground nuclear test on Tuesday despite strong and sustained opposition from the international community.

The DPRK said the test was part of the country's "practical measures of counteraction" to defend its security and sovereignty against hostile USpolicies, which it claimed have violated DPRK's "rights to launch satellite for peaceful purposes," the official KCNA news agency said in a statement.

The KCNA also said the test was done in a "safe and perfect" way with a smaller and light A-bomb unlike the previous ones, yet with great explosive power, posing no damage to "the surrounding ecological environment."

The statement has finally confirmed the conjecture of the international community that linked an earthquake occurred Tuesday in the DPRK with Pyongyang's plan for a fresh nuclear test, which has been strongly opposed.

Earlier in the day, China's Earthquake Networks Center reported a 4.9- magnitude temblor with a depth of "zero" km occurred in the DPRK, which was considered a potential sign that the country has finally undertaken its third nuclear test.

The earthquake was detected at 11:57 local time (0257 GMT) in the DPRK's Kilju County, North Hamkyung Province, where a nuclear test site is located.

Comprehensive Nuclear Test Ban Treaty Organization (CTBTO), the Vienna-based monitoring agency, said early Tuesday that the blast "location is roughly congruent with the 2006 and 2009 DPRK nuclear tests."

The South Korean Defense Ministry put the nuclear explosion at 6 to 7 kilotons, while an earlier report from the Yonhap news agency said the ministry calculated the blast at 10 kilotons.

South Korea's military has been immediately put on heightened alert to deter potential cross-border provocations following the reported DPRK test.

Meanwhile, the country's outgoing President Lee Myung-bak has convened a national security meeting to discuss the development.

A South Korean official said the DPRK has notified the United States and China of its nuclear test plan a day earlier.

Shortly after the DPRK's official confirmation of the test, the South Korean government condemned the test, calling it a "clear violation" of UN resolutions.

"In accordance with our consistent and firm principle that we will not tolerate North Korea (DPRK)'s nuclear tests, the government will seek all measures, including those at the level of UN Security Council, to deter North Korea's nuclear ambition," national security adviser Chun Yung-Woo told reporters after an emergency security meeting called by President Lee.

South Korea's President-elect Park Geun-hye, who will replaces Lee later this month, "strongly condemned" the test, and vowed to make joint efforts with the international community to denuclearize its northern neighbor, her spokeswoman Cho Yoon-sun told reporters on Tuesday.

"North Korea must strictly abide by all UN resolutions and keep its pledge of denuclearization," Park, who campaigned on improving inter-Korean relations, was quoted by Cho as saying.

Both Lee and Park are scheduled to meet to discuss the nuclear test and security situation.

South Korean Foreign Minister Kim Sung-hwan, meanwhile, held a telephone conversation with US Secretary of State John Kerry. They agreed to make a "swift and unified" response at the UN Security Council, which is expected to call an emergency meeting.

In Washington, US President Barack Obama also condemned Tuesday's nuclear test. While describing the blast as "highly provocative," he urged the international community to come up with "further swift and credible action."

He said in a statement issued by the White House that the test "undermines regional stability, violates North Korea's obligations under numerous United Nations Security Council resolutions."

Obama said the DPRK's latest move contravenes its commitments under the Sept. 19, 2005 Joint Statement of the Six-Party Talks on the DPRK's nuclear programs and "increases the risk of proliferation."

Japanese Prime Minister Shinzo Abe said at the lower house that the nuclear test is "regrettable" and violated relevant UN resolutions.

Abe said his government is mulling imposing unilateral sanctions on the DPRK over the nuclear test.

The Chinese Foreign Ministry said on Tuesday that it firmly opposed DPRK's nuclear test and urged Pyongyang to honor its denuclearization commitment and not take any actions that would worsen the situation on the Korean Peninsula.

The DPRK recently vowed to proceed with missile and nuclear tests targeting the United States, its "sworn enemy," to protest the UN condemnation over its Dec. 12 rocket launch.

Pyongyang conducted similar nuclear tests in 2006 and 2009.