Hours after North Korea claimed to have tested a miniaturised hydrogen nuclear device on Wednesday, South Korean intelligence reportedly said that the device may not be an H-bomb.

South Korea's meteorological agency Korean Meteorological Administration said it did not detect any radiation from the test, Reuters cited a report by South Korea's Yonhap News Agency.

Earlier on Wednesday, Pyongyang announced that the country successfully tested a hydrogen bomb in "self-defence against the US having numerous and humongous nuclear weapons". A 5.1 magnitude "man-made" earthquake was detected at 10 am Seoul time (6:30 am IST), purportedly caused by the nuclear test.

"If there's no invasion on our sovereignty we will not use nuclear weapon... This H-bomb test brings us to a higher level of nuclear power," CNN quoted the North Korean state news agency as saying.

The announcement soon triggered critical reactions from the United States, the United Nations, and other countries around the world.

John Carlson -- former head of Australia's nuclear safeguards and non-proliferation office -- also reserved his doubts about Pyongyang's claims to have tested "miniaturised hydrogen bomb", according to The Guardian.

Carlson said developing a miniaturised version of a thermonuclear weapon would be a "major challenge" for North Korea as it requires expertise. He said Pyongyang may have developed a lower-yield "boosted explosion" resulting in partial fusion of tritium -- an isotope of hydrogen.

"I think it's feasible that they've done that and they're claiming that they've done more," Carlson said.

South Korean President Park Geun-Hye addressed an emergency meeting of National Security Council on Wednesday to discuss North Korea's claims.

"The test is not only a grave provocation to our national security but also a threat to our future ... and a strong challenge to international peace and stability," she said, according to The Guardian.

Earlier, the National Security Council (NSC) of the White House issued a statement condemning "any violation of UN Security Council Resolutions".

"We are aware of seismic activity on the Korean Peninsula in the vicinity of Korean nuclear test site...but cannot confirm" claims of a nuclear test, the statement said.

Japanese Prime Minister Shinzo Abe, Australian Foreign Minister Julie Bishop and the UK's Secretary of State for Foreign and Commonwealth Affairs Philip Hammond also denounced the nuclear test.

"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," AFP quoted Abe as saying.

"If North Korean H-bomb test reports are true, it is a grave breach of UNSC resolutions and a provocation which I condemn without reservation," Hammond tweeted.

"Today's nuclear test confirms North Korea's status as a rogue state and a continuing threat to international peace and security," AAP quoted Bishop as saying.

French President Francois Hollande's office, in a statement, also criticised North Korea's H-bomb test and called for a "strong reaction from the international community", according to Reuters. It also reportedly called the test "an unacceptable violation of (UN) Security Council resolutions".

China's Xinhua state news agency criticised the test as running counter to the goal of the "denuclearisation" of northeast Asia, and called it "undesirable and unwise".

The United Nations Security Council (UNSC), which approved sanctions against North Korea after its third nuclear test in 2013, said it will meet later on Wednesday to discuss the latest move by Pyongyang, according to reports citing sources.