North Korea announced on Wednesday it had successfully carried out its first hydrogen bomb test, a development that, if confirmed, would mark a stunning step forward in its nuclear development.

“The republic’s first hydrogen bomb test has been successfully performed at 10:00am on January 6, 2016, based on the strategic determination of the Workers’ Party,” Korean Central Television (KCTV) announced.

“We have perfectly succeeded in testing our first hydrogen bomb,” an anchor said on North Korean state TV. “It was one hundred percent capable from our own wisdom, technology, and power. We have now scientifically test-proved a miniaturized hydrogen bomb.”

Before North Korea’s announcement, a magnitude 5.1 artificial earthquake was detected near the North’s Punggye-ri nuclear test site. The tremblor was detected about 12 miles ENE of Sungjibaegam and the South Korean weather agency said indications were that it was “artificial.”

The North Korean newscaster also claimed that the “perfectly conducted experimental hydrogen bomb test did not create any negative impact on the ecology in the area.”

“Heaven and earth are shaking because of this historic event,” she said. “It is our legal right as a sovereign nation to own hydrogen bomb for justice as we stand against the U.S., which is the culprit of invasion and who is looking for every opportunity to attack us with its vast pool of murderous nuclear weapons.”

The U.S. and Japan have called for an emergency meeting of the United Nations Security Council after North Korea claimed to have successfully tested a hydrogen bomb, a senior UN official told ABC News.

“U.S. Forces Korea is aware of reports on North Korea’s nuclear test today,” a U.S. Pacific Command spokesman said. “We remain vigilant and are fully committed to working closely with our Republic of Korea ally to maintain security on the peninsula.”

“We are monitoring and continuing to assess the situation in close coordination with our regional partners,” Price said in a statement.

One official said the U.S. doesn’t believe that North Korea has the capability for a hydrogen bomb but can’t be certain until testing is conducted.

If true, the hydrogen bomb test would be North Korea’s fourth nuclear test since 2006 and mark a huge advance in its nuclear capabilities.

North Korea criticized by China, Japan and South Korea for testing hydrogen bomb

The announcement by North Korea was swiftly criticised by world leaders with South Korea calling it “a grave provocation to our national security”. South Korea’s vice foreign minister, Lim Sung-nam, said during an emergency meeting that North Korea’s “provocation is in clear violation of Security Council resolutions and a serious challenge to international peace and security.”

China plans to summon North Korea’s ambassador in Beijing to the Foreign Ministry to lodge a strong protest, spokeswoman Hua Chunying told reporters at a daily briefing Wednesday.

Japanese Prime Minister Shinzo Abe condemned the alleged test, saying it was a “serious threat” to Japan’s security and “clearly violates the UNSC resolution and is a serious challenge to the nuclear non-proliferation efforts.”

The U.N. Security Council will meet Wednesday morning in New York for a closed-door session to discuss North Korea’s claim that it tested a hydrogen bomb, diplomats have told CNN.