South Korean and U.S. troops began naval drills Monday in a show of force partly directed at North Korea amid signs that Pyongyang will soon carry out a threat to conduct its third atomic test.

The region is also seeing a boost in diplomatic activity focused on North Korea's announcement last month that it will conduct a nuclear test to protest international sanctions toughened over Pyongyang's long-range rocket launch in December.

Pyongyang's two previous nuclear tests, in 2006 and 2009, both occurred after it was slapped with increased sanctions for similar rocket launches. The U.S., South Korea and other countries have urged North Korea to scrap its nuclear test plans or face grave consequences.

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.

"We assess that North Korea has almost finished preparations for conducting a nuclear test anytime and all that's left is North Korea making a political decision" to do so, Defense Ministry spokesman Kim Min-seok told reporters Monday.

The spokesman said he couldn't disclose further details because they would involve confidential intelligence affairs. Recent satellite photos showed North Korea may have been sealing the tunnel into a mountainside where a nuclear device could be exploded.

International community wants nuclear test scrapped

Meanwhile, diplomats are meeting to find ways to persuade North Korea to scrap its nuclear test plans. New U.S. Secretary of State John Kerry and his South Korean counterpart Kim Sung-hwan held a telephone conversation Sunday night and agreed to sternly deal with any possible nuclear provocation by North Korea, according to Seoul's Foreign Ministry.

South Korea on Sunday also sent its top nuclear negotiator to China, the North's main ally and aid benefactor, for talks, the ministry said in a statement.

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.

North Korea also says it has the sovereign right to launch rockets to send satellites into orbit under a space development program; the U.S. says the December launch was a disguised test of banned missile technology.

North Korea threatened by U.S.

On Monday, South Korean and U.S. militaries kicked off three-day 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.

North Korean state media on Saturday described the drills as a joint exercise for a pre-emptive attack on the country. North Korea has said similar things when South Korea and the U.S. conducted other drills. The allies have repeatedly said they have no intention of attacking the North.