The Latest on North Korea's nuclear test (all times local):

1:50 p.m.

North Korea says it has successfully conducted a nuclear explosion test aimed at examining the power of its nuclear warheads.

North Korea's state TV said Friday that the test elevated the country's nuclear arsenal and is part of its response to the international sanctions following its earlier nuclear test and long-range rocket launch in January and February.

North Korea says it will continue to take efforts to strengthen the quantity and quality of its nuclear weapons.

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12:55 p.m.

China says the Ministry of Environmental Protection has activated a contingency plan to begin monitoring radiation levels in provinces bordering North Korea, but says radiation levels are normal.

In Japan, meanwhile, two T-4 trainer aircraft took off from Hyakuri Air Base northeast of Tokyo, carrying a special container to collect air samples for analysis of possible radioactive materials.

Tokyo Gov. Yuriko Koike says Japan's capital city is also testing water samples and monitoring radiation levels in the air to examine possible impact from the North Korean nuclear test.

She told reporters: "I will protect the safety of Tokyo residents."

South Korea says North Korea on Friday conducted its fifth atomic test, producing its biggest-ever explosive yield, after monitors detected artificial seismic waves from a quake measuring a magnitude 5.

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12:50 p.m.

Japan's Prime Minister Shinzo Abe told reporters Friday that "there is a possibility that North Korea has forced a nuclear test," citing the temblor showing wave patterns from a non-seismic source.

He says: "If North Korea did conduct a nuclear test, it is absolutely not acceptable, and we must lodge a strong protest."

Japan's Foreign Minister Fumio Kishida also confirmed that Japan Meteorological Agency has detected shaking patterns that are not from a naturally occurred earthquake.

The meteorological agency detected a magnitude 5.3 shaking in North Korea, near the country's nulear test facility.

NHK says Japan's Nuclear Regulation Authority is now analyzing radiation levels at monitoring stations nationwide to see if there is any change.

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12:45 p.m.

South Korean President Park Geun-hye has strongly condemned North Korea's nuclear test, saying in a statement that it showed the "fanatic recklessness of the Kim Jong Un government as it clings to a nuclear development."

Kim is the North Korean leader.

Park's office says she spoke in Laos with President Barack Obama about the test Friday morning, but didn't immediately reveal more details.

Park says South Korea will employ all available measures to put more pressure on North Korea.

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12:35 p.m.

A spokesman for the U.S. National Security Council, Ned Price, says Washington is aware of seismic activity on the Korean Peninsula in the vicinity of a known North Korean nuclear test site.

He says: "We are monitoring and continuing to assess the situation in close coordination with our regional partners."

South Korea says North Korea on Friday conducted its fifth atomic test, producing its biggest-ever explosive yield, after monitors detected artificial seismic waves from a quake measuring a magnitude 5.

The U.S. Geological Survey called the seismic activity an "explosion" on its website.