North Korea Wednesday tested a hydrogen nuclear bomb, state news agency KCNA reported, marking the politically-isolated country's first nuclear test in three years, a move that received condemnation from the US, Britain and Japan among others.

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The report on the KCNA website came within hours of reports from various agencies that a large earthquake had been detected near a known North Korean nuclear test site.

According to KCNA, North Korea tested a miniaturized hydrogen nuclear bomb "in the most perfect manner," putting it in possession of hydrogen bomb capability, which it described as "the most powerful nuclear deterrent."

North Korea wanted what it called "the H-bomb of justice" as protection from the "ever-growing nuclear threat and blackmail by the US-led hostile forces," according to the statement on KCNA.

It would use the weapons only if its sovereignty was encroached upon, the statement on KCNA said, but would not roll back its nuclear development until the U.S. had dropped its "vicious, hostile" policy toward the isolated Communist state.

"The US is a gang of cruel robbers which has worked hard to bring even a nuclear disaster to the DPRK , not content with having imposed the thrice-cursed and unheard-of political isolation, economic blockade and military pressure on it for the mere reason that it has differing ideology and social system," according to the statement.

"The present-day grim reality clearly proves once again the immutable truth that one's destiny should be defended by one's own efforts," the statement went on. "Nothing is more foolish than dropping a hunting gun before herds of ferocious wolves."

This is North Korea's first nuclear test since February 2013 and the fourth it has conducted in all.

Japan reacts with anger

Japanese Prime Minister Shinzo Abe quickly condemned the test.

Abe said that Japan "absolutely cannot tolerate" a North Korean nuclear test, which he called a threat to Japan's security. Japan will make a firm response to North Korea's move, he added.

Shortly after, South Korean President Park Geun-hye said that North Korea would "pay the price" for its nuclear test, and that it would seek additional United Nations (UN) sanctions against its neighbor.

At the same time, according to South Korean news agency Yonhap, government officials noted that they needed to perform additional tests to ascertain that North Korea had actually tested a hyrdrogen nuclear device.

Intelligence sources told Yonhap that the device tested may not have been a H-bomb, while other Korean media cited defense sources as saying that North Korea may have added a small amount of hydrogen to the device tested..

Reuters reported that the Korean Meteorological Administration had detected no radiation from the apparent test.

Britain's Foreign Secretary Phillip Hammond said that such a test would be a "grave breach" of UN resolutions; North Korea is already under UN sanctions for having conducted previous nuclear tests, the first as early as 2006.

China's state news agency, Xinhua, wrote that the test was at odds with the goal of de-nuclearization, adding that any action that disrupted the stability of Northeast Asia was "undesirable and unwise." Xinhua is often read as a reflection of the thoughts of China's leaders.

Meanwhile, the US State Department condemned the apparent breach of UN sanctions.

Reuters reported that the UN Security Council would hold an emergency meeting on Wednesday at 11 am ET (4 p.m. GMT) to discuss the test, at the request of the US and Japan.

Earthquake shakes won

Word of the nuclear test emerged shortly after 9 a.m. SIN/HK, when the US Geological Survey (USGS) said it had detected a magnitude 5.1 earthquake about 49 kilometers (30 miles) a known North Korean nuclear test site, according to its coordinates. The USGS said that earthquake, near the site called Punggye-ri, was about 10km below the earth's surface.

The Korean Meteorological Administration, meanwhile, said that it detected the epicentre of the quake at a depth of "0 kilometers." It put the magnitude at 4.2.

First Published:Jan 6, 2016 9:58 AM IST