North Korea said it set off a hydrogen bomb Sunday in its sixth nuclear test, which judging by the earthquake it set off appeared to be its most powerful explosion yet.

Its state-run media said the country successfully tested a hydrogen bomb meant for an intercontinental ballistic missile.

The Joint Chiefs of Staff in Seoul says in a statement South Korea’s military has strengthened monitoring and readiness and is mulling a variety of possible responses that could be pushed together with its ally the U.S.

South Korea’s weather agency and the Joint Chiefs of Staff said an artificial 5.7 magnitude quake occurred at 12:29 p.m. local time, in Kilju, northern Hamgyong province, the site where North Korea has conducted nuclear tests in the past. Seoul officials revised their earlier estimate of 5.6 magnitude quake. The U.S. Geological Survey called the quake an explosion with a magnitude 6.3.

South Korea’s presidential office says the security chiefs for Seoul and Washington have spoken following North Korea’s sixth nuclear test.

The office says U.S. National Security Adviser H.R. McMaster spoke with his South Korean counterpart, Chung Eui-yong, for 20 minutes in an emergency phone call about an hour after the detonation.

South Korea’s presidential office said it will hold a National Security Council meeting chaired by President Moon Jae-in. South Korea’s military said it has strengthened its monitoring and readiness while mulling a variety of possible responses that could be executed in collaboration with the U.S.

Japan confirmed that North Korea conducted a nuclear test, Foreign Minister Taro Kono said. “It is absolutely unacceptable if North Korea did force another nuclear test, and we must protest strongly,” Prime Minister Shinzo Abe said.

With the testing of a hydrogen bomb, North Korea is provoking the international community and defying the toughest set of sanctions ever imposed by the U.N., reports CBS News’ Pamela Falk.

To date, the international community has been unanimous in condemning the nuclear and ballistic missile tests, but any further measures have been opposed by China and Russia.

China’s foreign ministry issued a statement saying the Chinese government “strongly opposes” and “condemns the test.”

The quake came just hours after North Korea’s state-run media claimed the developed the ability to load a hydrogen bomb onto a new intercontinental ballistic missile.

North Korea conducted two nuclear tests last year, the last nearly a year ago, on the September 9 anniversary of the nation’s founding. It has since maintained a torrid pace in weapons tests, including its first two intercontinental ballistic missiles test in July. Last month, North Korea fired a potentially nuclear-capable midrange missile over northern Japan.