Unlike previous North Korean tests, the device fired on Tuesday had greater miniaturization and was lighter, the agency said, suggesting it was more suitable for deployment on a missile. South Korea’s Defense Ministry estimated the yield of the device at 6-7 kilotons.

Seismic activity was detected in North Korea at 11:58 a.m. Korean time (02:58 GMT) on Tuesday, fuelling suspicions the country had carried out an underground nuclear test, as the area is not earthquake-prone.

"The event shows clear explosion-like characteristics and its location is roughly congruent with the 2006 and 2009 DPRK (North Korea) nuclear tests," the Comprehensive Nuclear Test Ban Treaty Organization’s Executive Secretary Tibor Toth said in a statement on Tuesday.

“If confirmed as a nuclear test, this act would constitute a clear threat to international peace and security, and challenges efforts made to strengthen global nuclear disarmament and non-proliferation, in particular by ending nuclear testing,” Toth added.

The quake’s epicenter was just one kilometer deep, and was close to a known nuclear test site in North Korea. Russian emergency service officials and forecasters have measured radiation levels in the Primorye Territory, which shares a border with North Korea, but reported “the daily average indicators have not changed.”

Global Reaction

The test has been condemned by South Korea, Japan, the United Kingdom, Russia, among other countries, as well as NATO and UN Chief Ban Ki-moon.

“Such behavior … deserves condemnation and an adequate reaction from the international community,” Russia’s Foreign Ministry said.

China, North Korea's major ally, expressed “firm opposition” to the nuclear test and called for “peace” and “stability" on the Korean Peninsula. North Korea notified the United States and China in advance that it would carry out a nuclear test, South Korea's Yonhap news agency reported.

Ramin Mehmanparast, spokesman for the Iranian Foreign Ministry, itself frequently called upon to defend Tehran's nuclear program, said "nuclear arms need to be destroyed", but added "all countries should have the right to make use of nuclear activities for peaceful purposes," AFP reported.

Russia's Foreign Minister Sergei Lavrov said: “[We] consider it important to urgently resume six-party talks on the Korean nuclear problem.”

Russia’s Deputy Foreign Minister Igor Morgulov, US Special Representative for North Korea Policy Glyn Davies and Shinsuke Sugiyama, Japan's chief envoy to the six-party talks on North Korea's nuclear weapons program, “discussed the situation on the Korean Peninsula in connection with the new nuclear test,” Russia’s Foreign Ministry said in a statement. No further details were reported.

“North Korea’s nuclear weapons and ballistic missile programs constitute a threat to US national security and to international peace and security,” US President Barack Obama said in a statement released by the White House.

The West has repeatedly alleged North Korea’s ultimate aim it is to deploy intercontinental ballistic missiles carrying nuclear warheads. North Korea claims it merely wants to develop space rockets to put satellites into orbit.

More Sanctions?

The UN Security Council is to convene an emergency meeting at 14:00 GMT on Tuesday.

North Korea vowed on January 23 to strengthen its military capabilities and step up its controversial nuclear program. The announcement came as an apparent response to the UN Security Council’s renewed sanctions on the country after it carried out a long-range rocket launch in December.

The latest atomic test will likely provoke a firmer reaction from Russia and China than previous ones, said Konstantin Asmolov, a Korean expert with the Institute of the Far East.