Updated at 7:24 a.m. Eastern.
North Korea fired artillery rounds toward its disputed sea border with South Korea on Wednesday, prompting a barrage of warning shots from the South's military and raising tensions on the divided peninsula.
No casualties or damage were reported, and analysts said the volley which the North announced was part of a military drill was likely a move by Pyongyang to highlight the need for a peace treaty to formally end the Korean War.
North Korea fired about 30 artillery rounds into the sea from its western coast and the South immediately responded with 100 shots from a marine base on an island near the sea border, an officer at the Joint Chiefs of Staff in Seoul said. The North said it would continue to fire rounds.
He said the North's artillery fire landed in its own waters while the South fired into the air. The officer spoke on condition of anonymity because of department policy.
The western sea border drawn by the American-led U.N. Command at the close of the 1950-53 Korean War is a constant source of tension between the two Koreas, with the North insisting the line be moved farther south.
Navy ships of the two Koreas fought a brief gun battle in November that left one North Korean sailor dead and three others wounded. They engaged in similar bloody skirmishes in 1999 and 2002.
North Korea issued a statement later Wednesday saying it had fired artillery off its coast as part of an annual military drill and would continue doing so.
Such drills "will go on in the same waters in the future," the General Staff of the (North) Korean People's Army said in a statement carried by the official Korean Central News Agency.
The North fired more shots later Wednesday, but South Korea didn't respond, a Defense Ministry official said, also requesting anonymity due to department policy.
The exchange of fire came two days after the North designated two no-sail zones in the area, including some South Korean-held waters, through March 29.
The North has sent a series of mixed signals to the South recently, combining offers of dialogue on economic cooperation with military threats, including one this month to destroy South Korea's presidential palace. South Korean Defense Minister Kim Tae-young, meanwhile, angered Pyongyang by saying Seoul's military should launch a pre-emptive strike if there was a clear indication the North was preparing a nuclear attack.
South Korea's Defense Ministry sent the North's military a message Wednesday expressing serious concern about the firing and saying it fostered "unnecessary tension" between the two sides.
It also urged the North to retract the no-sail zones, calling them a "grave provocation" and a violation of the Korean War armistice. The war ended with a truce, but not a formal peace treaty.
Separately, South Korea's point man on North Korea criticized Pyongyang for raising tension near the sea border.
"This kind of North Korean attitude is quite disappointing," Unification Minister Hyun In-taek told a security forum in Seoul.
South Korea's Yonhap news agency said it was the first time that North Korea has fired artillery toward the sea border. The Joint Chiefs of Staff officer said the North Korean artillery shells were believed to have fallen into the no-sail zones about 1.75 miles (3 kilometers) north of the maritime border.
Top South Korean presidential secretary Chung Chung-kil convened an emergency meeting of security-related officials on behalf of President Lee Myung-bak, who was making a state visit to India, according to the presidential Blue House. It said Lee was informed of the incident.
Yoo Ho-yeol, a professor of North Korean studies at Korea University in South Korea, said the North's action was aimed at highlighting the need for a peace treaty to formally end the Korean War by showing that the peninsula is still a war zone.
"It's applying pressure on the U.S. and South Korea," Yoo said. He said North Korea also was expressing anger over South Korea's lukewarm response to a series of recent gestures seeking dialogue.
Earlier this month, North Korea called for the signing of a peace treaty and the lifting of sanctions as conditions for its return to stalled nuclear disarmament talks it quit last year.
The U.S. and South Korea, however, brushed aside the North's demands, saying they can happen only after it returns to the disarmament negotiations and reports progress in denuclearization.
Despite the exchange of fire, the capitals of the two Koreas were calm.
North Koreans in Pyongyang wearing thick winter coats walked briskly through the streets while a female police officer directed traffic and a crowded tram passed by, according to footage shot by broadcaster APTN.
The military tensions had little effect on South Korean financial markets. Seoul's benchmark stock index fell less than 1 percent, while South Korea's currency, the won, rose against the U.S. dollar.