North Korea is threatening a "physical response" to scheduled U.S.-South Korean naval maneuvers beginning Sunday in the Sea of Japan.
The outburst Friday was the latest shot in a war of words that eclipsed other issues at the ASEAN Regional Forum in Hanoi.
On Friday U.S. Secretary of State Hillary Clinton described North Korea as an "isolated and belligerent" regime engaged in a "campaign of provocative, dangerous behavior." Earlier this week, she announced new economic sanctions against Pyongyang.
North Korean spokesman Ri Tong Il responded furiously at the 27-member forum, describing the planned exercise as a threat to his nation's sovereignty and a reminder of 19th century gunboat diplomacy.
He said, "There will be a military response against the threat imposed by the United States militarily."
Clinton defended the military exercise, saying it would demonstrate America's commitment to the defense of South Korea.
Why hold exercises?
The exercise was organized in response to the sinking in March of a South Korean warship, the Cheonan, killing 46 sailors. An international investigation concluded the ship had been torpedoed by a North Korean submarine, but the North denies any responsibility.
A statement prepared for approval by all the countries at the forum expressed concern about the sinking but did not specifically blame any country.
The spat between the United States and North Korea diverted attention from other issues before the 27-member forum, which is hosted by the 10-member Association of Southeast Asian Nations and embraces other interested countries including China, Russia, Japan, North and South Korea and the United States.
Those issues included promised elections in Burma, the first in 20 years. ASEAN members have been pressing Burma to ensure the elections are free and fair.
Clinton repeated that message Friday and urged the ASEAN members to remind Burma of its obligations to comply with international non-proliferation measures. She expressed concern about reports that North Korea is helping Burma develop its own nuclear program.
Clinton also urged regional leaders to resolve long-standing disputes over a chain of islands in the South China Sea, especially between China and Vietnam. She said Washington is concerned the disputes will interfere with maritime commerce.
Clinton said U.S. President Barack Obama will invite ASEAN leaders to Washington for a summit to take place later this year.
Some information for this report was provided by AP, AFP and Reuters.