New U.S. Secretary of State John Kerry and his South Korean counterpart have agreed to make sure North Korea is punished if it carries out its threat to conduct a nuclear test. Amid signs that such a test is coming, South Korea’s president also speculated that Pyongyang may detonate multiple atomic devices simultaneously.

Under a U.N. Security Council resolution last month condemning a North Korean long-range rocket launch that the U.N. and others call a disguised test of banned missile technology, Pyongyang is subject to new sanctions if it detonates its third nuclear device since 2006.

North Korea announced last month that it will conduct a nuclear test to protest the toughened sanctions over its December launch, which delivered a satellite into orbit.

During a phone conversation between Kerry and South Korean Foreign Minister Kim Sung-hwan, “there was agreement that … if the DPRK continues its provocative behavior and takes further steps, that there must be further consequences,” State Department spokeswoman Victoria Nuland said Monday. DPRK is the acronym for North Korea’s official name, the Democratic People’s Republic of Korea.

North Korea may simultaneously conduct multiple nuclear tests as it tries to create a warhead small enough to fit atop a long-range missile, South Korean President Lee Myung-bak told the South Korean newspaper, Chosun Ilbo, in an interview published Tuesday. Lee did not say whether his comments were based on intelligence findings.

A North Korean nuclear test “seems to be imminent,” South Korea’s U.N. Ambassador Kim Sook said Monday at a news conference at U.N. headquarters in New York.

He said there are “very busy activities” taking place at North Korea’s nuclear test site “and everybody’s watching.” The ambassador said he expects the Security Council to respond with “firm and strong measures” in the event of a nuclear test.

North Korea has denounced sanctions over its rocket launches, saying it has the sovereign right to use rockets to send satellites into orbit under a space development program.

Pyongyang’s two previous nuclear tests, in 2006 and 2009, both occurred after it received international criticism for similar rocket launches. As it issued its most recent punishment, the Security Council ordered North Korea to refrain from a nuclear test or face “significant action.”

North Korea’s state media said Sunday that at a high-level Workers’ Party meeting, leader Kim Jong Un issued “important” guidelines meant to bolster the army and protect national sovereignty. North Korea didn’t elaborate, but Kim’s guidelines likely refer to a nuclear test and suggest that Pyongyang appears to have completed formal procedural steps and is preparing to conduct a nuclear test soon, according to South Korean analyst Hong Hyun-ik.

Recent satellite photos show North Korea may have been sealing the tunnel into a mountainside where a nuclear device could be exploded.

The Washington-based Institute of Science and International Security has released satellite imagery by Digital Globe from Jan. 28 that shows activity at tunnels at North Korea’s nuclear testing site. “Although the images do not reveal whether a test is imminent, the ongoing activity at the site justifies concern that a test will soon occur,” the private nonproliferation group said.

On Monday, the South Korean and U.S. militaries kicked off three days of exercises off the Korean Peninsula’s east coast that involve live-fire exercises, naval maneuvers and submarine detection drills.

South Korea’s Joint Chiefs of Staff said the maneuvers are part of regular joint military training that the allies had scheduled before the latest nuclear tensions began. But the training, which involves a nuclear-powered American submarine, could still send a warning against possible North Korean provocation, a South Korean military official said, requesting anonymity because of department rules.

Pyongyang’s state media said the drills showed that the U.S. and South Korea have been plotting to attack North Korea and increased the danger of a war on the divided peninsula. “The dark cloud of war is approaching to the Korean Peninsula,” North Korea’s official Uriminzokkiri website said in a commentary.

North Korea has said similar things about previous drills; the allies have repeatedly said they have no intention of attacking the North.

North Korea says U.S. hostility and the threat of American troops in South Korea are important reasons behind its nuclear drive. The U.S. stations about 28,500 troops in South Korea, a legacy of the 1950-53 Korean War, which ended with an armistice, not a peace treaty.

(AP)