North Korea said on Wednesday that it had conducted a successful hydrogen bomb test, hours after a 5.1 magnitude earthquake was detected near a known test site in the country.

Before the announcement, which was made on state television, South Korea's meteorological agency said the North was likely to have conducted a nuclear test based on data around the earthquake.

South Korea's presidential office held an emergency meeting and later said the government would take all possible measures in response to the apparent test, including possible UN sanctions.

"Our government strongly condemns North Korea ignoring repeated warnings from us and the international community and pushing ahead with the fourth nuclear test, which clearly violated the UN resolutions," Cho Tae-yong, a senior security official at the South Korean presidential office said.

The US government said it could not confirm a nuclear test but that it would respond appropriately to what it called North Korean "provocations" and that it would continue to protect its allies in the region.

"We have consistently made clear that we will not accept [North Korea] as a nuclear state," a State Department statement said.

Japanese Prime Minister Shinzo Abe called the announcement a threat to his nation's safety.

Abe told reporters: "We absolutely cannot allow this, and condemn it strongly."

British Foreign Secretary Philip Hammond said any nuclear test would be a "provocation which I condemn without reservation" and a "grave breach" of UN Security Council resolutions.

The United States Geological Survey (USGS) also detected the quake that South Korea said was 49 km (30 miles) from the Punggye-ri site where the North has conducted nuclear tests in the past.

"We suspect a man-made earthquake and are analyzing the scale and epicenter of the quake," a Korea Meteorological Administration official told the Reuters news agency by phone.

While the USGS put the depth of the earthquake at 10km, the South Korean agency said it was near the surface. The earthquake was detected just after 10am Seoul time (0100 GMT).

Al Jazeera's Adrian Brown, reporting from Beijing, said that North Korea was moving one step nearer to creating a nuclear warhead.

"There will be a good deal of tension once more on the Korean peninsula today. The big question of course is why North Korea has done this now," he said.

Brown added the test could partly be seen as a defensive response to US-South Korean military exercises, as well as a reaction to UN sanctions the country is under for its nuclear and missile programmes.

"When North Korea last tested a nuclear device in 2013, that was as a direct consequence of the sanctions that had been imposed by the United Nations," Brown said.

North Korea has so far conducted three nuclear tests - in 2006, 2009 and 2013 - all at Punggye-ri.

The 2013 test registered at 5.1 on the USGS scale.

It is not yet known if Pyongyang has successfully miniaturized a nuclear device small enough to be used as a warhead on a ballistic missile, but the likelihood of the isolated country successfully doing so increases with each test.

PHOTO CAPTION

North Korean leader Kim Jong Un inspects the completed Mirae Scientists Street, in this undated photo released by North Korea''s Korean Central News Agency (KCNA) in Pyongyang.

Al-Jazeera