SEOUL, South Korea (AP) — The Latest on North Korea's nuclear test (all times local):

2:40 p.m.

China has come out in strong opposition of North Korea's fifth nuclear test, a key denunciation for Pyongyang by its economic lifeline and only major ally.

The Foreign Ministry issued a statement Friday criticizing North Korea for carrying out a test with "disregard" for international objections.

North Korea said Friday that it had detonated a warhead, hours after South Korean officials said they had detected seismic activity near a known nuclear test site.

China has provided cover to North Korea from worldwide denunciations of its nuclear program. But it toughened its line after Pyongyang carried out long-range missile tests earlier this year, restricting exports of jet fuel into the country and banning some mineral imports.

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2:15 p.m.

President Barack Obama has been briefed about the report of seismic activity near a nuclear facility in North Korea.

South Korean officials say it was indeed a nuclear test, the fifth by the North.

Obama returned to Washington from a trip to Asia just before 1 a.m. EDT Friday. His press secretary, Josh Earnest, says Obama received the briefing aboard Air Force One from his national security adviser, Susan Rice.

Earnest says Obama also consulted with South Korean President Park Geun-hye and Prime Minister Shinzo Abe of Japan in separate phone calls.

Earnest says Obama reiterated the unbreakable U.S. commitment to the security of America's allies in Asia and around the world. The spokesman says Obama indicated he would continue to consult America's allies and partners in the days ahead "to ensure provocative actions from North Korea are met with serious consequences."

The spokesman for the State Department, John Kirby, says Secretary of State John Kerry has been briefed on the matter and that officials are monitoring and assessing the situation.

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2 p.m.

North Korea's state TV says Friday's nuclear test "examined and confirmed" specific features of a nuclear warhead designed to be mounted on ballistic missiles. It says there was no radioactive leakage or adverse environmental impact caused by the test.

North Korea says the test shows the country is ready to hit back if provoked by enemies including the United States, and that it will continue its efforts to strengthen the quantity and quality of its nuclear weapons.

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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.