Update, 1:35 pm: China's Foreign Ministry has reacted to North Korea's claims that it tested a hydrogen bomb on Wednesday. It said Beijing "firmly opposes" the nuclear test, according to Xinhua.
"DPRK (Democratic People's Republic of Korea) should remain committed to denuclearization and stop any actions worsening the situation," China's foreign ministry spokeswoman said.
Pyongyang on Wednesday announced that it has "successfully" tested miniaturised hydrogen nuclear device, calling it a "historic event of the national significance".
The ruling Workers' Party of Korea issued a statement justifying its decision to develop and test H-bomb claiming that the US is "watching for a chance for attack on it with huge nukes of various types".
"The first H-bomb test was successfully conducted in Juche Korea at 10:00 on Wednesday, Juche 105 (2016), pursuant to the strategic determination of the WPK," the statement, translated and shared on Twitter by VOA News bureau chief, said.
"It was confirmed that the H-bomb test conducted in a safe and perfect manner had no adverse impact on the ecological environment," the statement said. It added that the test means "a higher stage of the DPRK's development of nuclear force".
"This test is a measure for self-defence the DPRK has taken to firmly protect the sovereignty of the country and the vital right of the nation from the ever-growing nuclear threat and blackmail by the US-led hostile forces and to reliably safeguard the peace on the Korean Peninsula and regional security," it added.
"The Korean Peninsula and its vicinity are turning into the world's biggest hotspot where a nuclear war may break out since they have been constantly stormed with all nuclear strike means of the U.S. imperialist aggressor troops, including nuclear carrier strike group and nuclear strategic flying corps," it added.
The statement further states that North Korea will not "suspend or dismantle nuclear development" unless the US rolls back "its vicious hostile policy toward the former".
"The army and people of the DPRK will steadily escalate its nuclear deterrence of justice both in quality and quantity to reliably guarantee the future of the revolutionary cause of Juche for all ages," it added.
Update, 10:37 am: National Security Council (NSC) of the White House said in a statement, "we are aware of seismic activity on the Korean Peninsula in the vicinity of Korean nuclear test site" but "cannot confirm" claims of nuclear test.
The statement further added that the US condemns "any violation of UN Security Council Resolutions", White House NSC spokesperson Ned Price said in a statement on Wednesday.
"North Korea conducted its first nuclear test in 2006 and, until today, has done so twice since, but we have consistently made clear that we will not accept it as a nuclear state," he added.
Japanese Prime Minister Shinzo Abe also condemned the test fire of the hydrogen bomb. "I strongly condemn this. The nuclear test that was carried out by North Korea is a serious threat to the safety of our nation and we absolutely cannot tolerate this," he told reporters, according to AFP.
Australian Foreign Minister Julie Bishop has also condemned North Korea and called the nuclear test "provocative and dangerous". "Today's nuclear test confirms North Korea's status as a rogue state and a continuing threat to international peace and security," she said in a statement, Australian Associated Press reported.
— China Xinhua News (@XHNews) January 6, 2016
North Korea said it conducted its first "successful" miniaturised hydrogen nuclear device test on Wednesday, which is suspected to have caused the 5.1 magnitude earthquake that was detected 19 kms from nuclear test site -- Punggye-ri.
"The republic's first hydrogen bomb test has been successfully performed at 10:00 am(12.30pm AEDT) on January 6, 2016, based on the strategic determination of the Workers' Party," a state television news reader announced, according to 9News.
North Korea said the test was "self-defence against the US having numerous and humongous nuclear weapons", the state television reported, according to The Guardian.
Quake monitoring agencies have indicated that the earthquake, which occurred at 10 am Seoul time (6:30 am IST), could be man-made.
"We suspect a man-made earthquake and are analysing the scale and epicentre of the quake with the geoscience and mineral resource institute of South Korea," The Guardian quoted a South Korean Meteorological Administration as saying.
The US Geological Survey recorded the depth of the quake at 10 kms and the South Korean agency recorded the epicentre 49 kms (30 miles) from the Punggye-ri site, where North Korea has conducted nuclear tests in the past, Reuters reported.
South Korean presidential office has called for an emergency meeting of the National Security Council.
Seismology experts in China said they have detected "man-made" earthquake in North Korea that could have been caused by an atomic explosion. The website of the China Earthquake Network Centre has described the earthquake as a "suspected explosion" that occurred at the depth of "zero kilometres and a magnitude of 4.9", The Guardian reported.
The chief cabinet secretary of Japan, Yoshihide Suga, said, "considering past cases, there is the possibility that this might be a nuclear test by North Korea", AFP reported.
North Korea has so far conducted three nuclear tests at Punggye-ri site; one in October 2006, another in May 2009 and the last one in February 2013. The nuclear test in 2013 was registered at 5.1 on the USGS scale.
The 2013 nuclear test attracted criticism from China and other nations. After the third underground test, the United Nations Security Council also approved a sanction against North Korea for violating its nuclear and missile proliferation activities.
Researchers at US-Korea Institute at Johns Hopkins University had in December said that North Korea was digging up a new tunnel at Punggye-ri. "While there are no indications that a nuclear test in imminent, the new tunnel adds to North Korea's ability to conduct additional detonations over the coming years if it chooses to do so," they had said then, according to AFP.
We are aware of seismic activity on the Korean Peninsula & have seen claims of a nuclear test. Statement: pic.twitter.com/WvyFkpJcZo
— WH National Security (@NSCPress) January 6, 2016