North Korea has issued fresh threats against South Korea, vowing retaliation for recent protests in Seoul that it says "hurt the dignity" of the leadership in Pyongyang.
The North's military threatened unspecified "immediate" action against the South if it did not apologize for the small Monday protests, during which effigies of North Korean leaders were burned.
The statement, read on state television Tuesday, included typically inflammatory language, vowing "sledge-hammer" revenge for what it said was the protesters' "monstrous criminal act."
"Our retaliatory action will start without any notice from now as such the thrice-cursed criminal act of hurting the dignity of the supreme leadership of North Korea is being openly committed in the heart of Seoul under the patronage of the puppet authorities."
The threat comes as North Koreans continue their two-day celebration of the birthday of late founding leader Kim Il Sung. Many had expected Pyongyang to mark the occasion with a provocative missile test, but the Monday anniversary passed without incident.
The South's defense ministry Tuesday called the new threats "regrettable." Spokesperson Kim Min-Seok said Seoul remains on alert for a possible missile test.
"It is still possible that North Korea will fire missiles. So we are constantly chasing up those kinds of missiles - Scud, Rodong, and Musudan missiles."
White House spokesperson Jay Carney on Monday welcomed Pyongyang's decision thus far to refrain from a missile test, but said Washington continues to monitor the situation closely in case of any provocation.
The White House also said President Barack Obama and South Korean President Park Geun-hye will meet in Washington on May 7 to discuss what it called the "North Korean threat," among other issues.
For weeks, Pyongyang has made repeated threats of possible nuclear attack -- including against the U.S. mainland. The situation topped the agenda of Secretary of State John Kerry's trip to the region, which he wrapped up Monday in Japan.
Kerry said Washington is willing to negotiate with North Korea for a peaceful resolution of tensions on the Korean peninsula, if Pyongyang takes steps toward abandoning nuclear weapons. But he also repeated the U.S. commitment to the defense of both South Korea and Japan.
Pyongyang has been angered by joint U.S.-South Korean military exercises that it sees as a prelude to an invasion of the North. Washington and Seoul have insisted the drills are defensive.
As those drills continue, officials said a U.S. military helicopter crashed Tuesday near the North Korean border. No fatalities were reported. It was not immediately clear what caused the crash.