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The attack is the latest provocation by the economically strapped communist regime in Pyongyang and followed the sinking in March of the South Korean warship Cheonan in the same region. An international investigation later concluded that a North Korean torpedo, probably fired from a mini-submarine, caused the sinking and the deaths of 46 sailors.

Tensions also have been raised over North Korea’s once-covert uranium enrichment program that was shown recently to three visiting American nuclear specialists.

North Korean state radio interrupted broadcasting at 7:28 p.m. local time for a statement from the Korean People’s Army Supreme Command that denounced the South Korean military for holding military exercises.

The statement claimed South Korea had launched artillery shells in North Korean waters and that its firing was designed to counter the attack. The statement said its artillery firing was an “immediate and powerful physical strike” and warned that a “merciless” counterstrike would follow any further maritime border violations.

A diplomat in Washington familiar with the region said the North Koreans fired the artillery in response to South Korean military exercises near the island. The diplomat said the exercises did not violate North Korean sovereignty because they were held on the South Korean side of the line separating the two countries.

“Our military fired artillery toward south of Yeonpyeong Island,” the diplomat said. “Therefore, North Korean assertions that their actions were in response to our military exercises were totally groundless.”

South Korea put its military forces on high alert after the incident, and South Korean President Lee Myung-bak convened an emergency Cabinet meeting to discuss a response to the attack. Mr. Lee said an “indiscriminate attack on civilians can never be tolerated.”

“Enormous retaliation should be made to the extent that [North Korea] cannot make provocations again,” Mr. Lee said.

In Seoul, a spokesman for Mr. Lee said the attack on Yeonpyeong was an “indisputable armed provocation” against South Korea.

“Such actions will never be tolerated,” the spokesman said, adding the North must take full responsibility for the incident.

Mr. Obama was awakened by Mr. Donilon about 4 a.m. and notified of the incident.

Later Tuesday morning, White House press secretary Robert Gibbs called on the North to “halt its belligerent action” and fully abide by the terms of the 1953 Armistice Agreement.

Bill Burton, White House deputy press secretary, told reporters aboard Air Force One en route to Kokomo, Ind., that Mr. Obama was “outraged” by the North’s actions.

In New York, U.N. Secretary-General Ban Ki-moon said he was deeply concerned about the escalation of tension on the Korean Peninsula and described the attack as one of the “gravest incidents since the end of the Korean War,” said Mr. Ban’s spokesman Martin Nesirky.

Mr. Ban, a former South Korean foreign minister, condemned the attack and called for immediate restraint.

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