North Korea successfully conducted a hydrogen nuclear bomb test on Wednesday, the North’s state media claimed, saying that it would continue to strengthen its nuclear deterrence so long as the U.S. maintains its hostile policy toward the communist state.

The North’s Korean Central Television issued a “crucial report” at 12:30 a.m., two hours before the world’s seismic analysis centers reported that a 5.1-magnitude artificial earthquake occurred in the North.

According to the European Mediterranean Seismological Center, the U.S. Geological Survey and China’s state seismic center, the earthquake’s epicenter was in Yangang Province in the country’s northeast. They said the earthquake took place at 10:30 a.m.

The North conducted underground nuclear tests in 2006, 2009 and 2013.

The North reportedly conducted the nuclear test without notifying China in advance, raising the possibility that the relations between the traditional allies would further deteriorate. China has long been against the North’s possession of nuclear arms.

A senior Seoul official said it would seek close cooperation with the U.S. and China over the issue, and that a U.N. Security Council session will open to discuss the issue.

Cheong Wa Dae convened a Security Council Session at noon.

“The artificial earthquake occurred near Punggye-ri, the North’s nuclear site. It has yet to be confirmed whether it was a nuclear test or not,” the official told reporters on the customary condition of anonymity. “We will analyze it to see if it is really a nuclear test. And then we will take due steps.”

Seoul’s Foreign Ministry’s division in charge of peninsular peace affairs has entered emergency mode. It has contacted related agencies from its U.S. and Chinese counterparts to confirm if the earthquake really was caused by a nuclear test.

The earthquake took place after a flurry of news reports said the North successfully conducted a test on a submarine-launched ballistic missile, which would pose a graver security challenge to South Korea and the U.S.

By Song Sang-ho (sshluck@heraldcorp.com)