Later Monday, 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. “Our patience has the limit.”
North Korea said similar things when South Korea and the U.S. conducted 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.
North Korea also has denounced sanctions over its rocket launches, saying it has the sovereign right to launch rockets to send satellites into orbit under a space development program.
North Korea's two previous nuclear tests are believed to have been explosions of plutonium devices, but experts say the North may use highly enriched uranium for its upcoming test. That is a worry to Washington and others because North Korea has plenty of uranium ore, and because uranium enrichment facilities are easier to hide than plutonium facilities are.
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, Seoul's Foreign Ministry said in a statement.
The chief nuclear envoys of South Korea and China met in Beijing on Monday and agreed that they would closely coordinate on ways to stop North Korea from conducting a nuclear test, according to Seoul's Foreign Ministry. China is North Korea's main ally and aid benefactor.
China has refused to say whether it was sending an envoy to North Korea or whether Pyongyang has informed Beijing about its plans for a nuclear test. China's Foreign Ministry on Monday reiterated Beijing's opposition to a test, though it did not mention North Korea by name.
“We call on all sides, under the current circumstances, to avoid taking measures which will heighten regional tensions. We hope all parties concerned can focus their efforts more on helping to ease tensions on the peninsula and throughout the region and jointly maintain peace and stability on the peninsula,” Chinese Foreign Ministry spokeswoman Hua Chunying said at a daily media briefing in Beijing.