U.S., allies warn North Korea against 'provocative' moves

(CNN) -- U.S. Secretary of State John Kerry and his South Korean and Japanese counterparts warned North Korea against any "provocative" moves Sunday ahead of a possible new nuclear bomb test by Pyongyang.

Opinion: For South Koreans, a familiar tone from Pyongyang

In a round of calls Sunday, Kerry, Japanese Foreign Minister Fumio Kishida and South Korea's Kim Sung-hwan all agreed the North must understand "that it will face significant consequences from the international community if it continues its provocative behavior," according to a summary of the calls from the U.S. State Department.

Inside a North Korean school in Japan

Earlier Sunday, North Korea announced that its leader, Kim Jong Un, "has made an important decision" that would strengthen the country. The brief statement on the state-run news agency KCNA provided no details, but it said the decision was made at a meeting of the reclusive Stalinist state's Party Central Military Committee.

Pyongyang threatens South Korea

Region braces for North Korea nuke test

Space race on Korean Peninsula

Across the Demilitarized Zone, South Korean President Lee Myung-bak called on his government to be prepared for a possible test. Lee paid a visit to the underground bunker that serves as the South's crisis management center, his press office reported.

South Koreans cast wary eyes to the North

North Korea has conducted two previous nuclear tests, in 2006 and 2009, and proclaimed itself a "nuclear state" in 2012. U.S. officials told CNN last week that the North appeared to be ready test another nuclear device "at any time."

U.S. analysts believe the 2006 test had a yield of about 1 kiloton -- comparable to the explosive power of about 1,000 tons of TNT -- while the second was roughly 2 kilotons, National Intelligence Director James Clapper told a Senate committee in 2012.

By comparison, the bomb the United States dropped on Hiroshima in 1945 was roughly 15 kilotons.

Where North Korea stands in its pursuit of a nuclear missile

The U.N. Security Council voted to tighten sanctions on Pyongyang in January, after the North launched a satellite aboard a long-range rocket in December.

The North Koreans responded by announcing they planned another nuclear test and more long-range rocket launches as part of a new phase of confrontation with the United States.