Seoul, South Korea (CNN) -- North Korea fired artillery toward its tense western sea border with South Korea on Tuesday, killing two South Korean marines, the South's Defense Ministry said.
Fifteen other South Korean soldiers were wounded, five of them seriously, defense officials said. Three civilians were injured in the attack.
About 100 rounds of artillery hit an inhabited South Korean island in the Yellow Sea after the North started firing about 20 p.m. local time, the Yonhap news agency said. Yonhap initially reported that 200 rounds had hit. The Defense Ministry said it could not confirm the number of rounds.
South Korea's military responded with more than 80 rounds of artillery and deployed fighter jets to counter the fire, defense officials said. Firing between the two sides lasted for about an hour.
The South Korean army also raised its alert condition, Yonhap said.
Images of plumes of smoke were quickly broadcast on Yonhap television from the island of Yeonpyeong, with some homes on fire. It was not immediately clear how much damage the artillery had done. The island has a large military garrison.
The island has a total of about 1,300 residents, a fisherman who lives on the island told Yonhap.
Some residents started fleeing for the South Korean mainland, which is about 145 kilometers [90 miles] away. Other residents were seeking shelter at schools.
The South Korean government immediately called an emergency meeting of its security ministers, meeting in a bunker under the presidential residence in Seoul.
South Korean President Lee Myung-bak ordered his ministers to take measures against an escalation of the situation, presidential spokeswoman Kim Hee-jung said, according to Yonhap.
"Take a stern response and carefully manage the situation from further escalating," the president said.
The United States quickly offered support.
"We are in close and continuing contact with our Korean allies," the White House said in a news release. "The United States strongly condemns this attack and calls on North Korea to halt its belligerent action and to fully abide by the terms of the Armistice Agreement."
"The United States is firmly committed to the defense of our ally, the Republic of Korea, and to the maintenance of regional peace and stability."
The North Korean fire came as the South's military conducted routine drills in waters off the island, which is about 10 kilometers (6 miles) from the North.
South Korea's annual Hoguk military exercises were to last nine days and include as many as 70,000 South Korean military personnel, according to the Stratfor global intelligence company.
The Yellow Sea has been a longstanding flashpoint between the two Koreas, but Tuesday's attack was an escalation in violence.
"Our navy was conducting a maritime exercise near the western sea border today. North Korea has sent a letter of protest over the drill. We're examining a possible link between the protest and the artillery attack," presidential spokeswoman Kim said, according to Yonhap.
"Marines were training in that area, including firing artillery, in the morning. But they were aiming south and southwest, not east or north," said a representative for the Joint Chiefs of Staff. "North Korea sent a telephone message at 8:20 a.m. to cease the drill. We did not stop the drill."
Yeonpyeong island is part of a small archipelago about 80 kilometers [49 miles] west of the South Korean port of Inchon, which serves Seoul, and is close to the tense Northern Limit Line, the maritime border between the two Koreas in the Yellow Sea.
A South Korean warship, the Cheonan was sunk in the area in March with the loss of 46 lives in a suspected North Korean torpedo attack.
North Korean artillery is extremely difficult to hit, because it is dug into coastal cliffs. Though the North has tested its artillery -- and tested anti-shipping missiles -- it has not fired artillery into South Korean territory in recent years.
One of North Korea's most potent threats is the hundreds of artillery barrels dug in along its demilitarized zone with South Korea and ranged on Seoul.
Yonhap television was covering the attack nonstop in South Korea, forgoing other news Tuesday. Meanwhile, state television in North Korea did not mention the attack.
The reason for the attack was unclear, but North Korea watchers had theories.
"I think they are very frustrated with Washington's response to their uranium program and think they think that Washington has almost given up on negotiations with North Korea," said Choi Jin-wook, senior researcher at the Korea Institute of National Unification.
"I think they realize they can't expect anything from Washington or Seoul for several months, so I think they made the provocation."
"I definitely think this is centrally directed form Pyongyang. This can't be done without orders from Pyongyang," he added.
Over the weekend, news broke that North Korea had showed off its uranium-enrichment facilities to a visiting U.S. scientist. Washington reacted by saying that the North's nuclear moves had been clear all along.
The United States said it would not dismiss restarting six-party talks aimed at denuclearizing the North. However, it said it would not return to negotiations unless North Korea showed good faith.
North Korea is desperately reaching for bargaining chips, experts say.
"They want food. They are starving to death. They are trying to make Seoul and Washington move. Otherwise, they are in big trouble," Choi Jin-wook of the Korea Institute for National Unification said before Tuesday's artillery attack. "And this is a transition period for the North Korean leadership; they need to provide gifts to the elite, but they don't have the resources."
Sanctions have been progressively placed on North Korea in response to a succession of nuclear and missile tests and the sinking of the South Korean warship in March.
Meanwhile, with national leader Kim Jong Il apparently in ailing health, his son Kim Jong Un is being raised to prominence in the isolated state, in what pundits see as a succession process.
Sharp tensions on the Korean Peninsula
Report: N. Korea fires on S. Korea