Seoul, South Korea (CNN) -- Hours after North Korea's deadly artillery attacks Tuesday, South Korea's president said "enormous retaliation" is needed to stop Pyongyang's incitement, but international diplomats urgently appealed for restraint. "The provocation this time can be regarded as an invasion of South Korean territory," President Lee Myung-bak said at the headquarters of the Joint Chiefs of Staff here, according to South Korea's Yonhap news agency. The incident -- in which two South Korean marines died -- is "the first direct artillery attack on South Korean territory since the Korean War ended in an armistice" in 1953, Yonhap reported.
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NEW: South Korea, U.S. agree to joint exercises
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President Obama, U.S. Rep. Boehner condemn North Korean action
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Two South Korean marines were killed; 15 civilians and soldiers hurt
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Pyongyang calls South's military drills "reckless military provocation"
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@placeholder also suspended promised flood aid to North Korea, Yonhap reported, citing an unnamed official.
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A senior U.S. defense official said South Korea had informed @placeholder prior to the training mission, and that "there's no reason North Korea should have been surprised by this firing of artillery."
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Sanctions have been progressively placed on @placeholder in response to a succession of nuclear and missile tests and the sinking of the South Korean warship in March.
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Countries that had been negotiating with @placeholder over its nuclear program issued swift reactions.
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The incident could reflect a more aggressive view of what a nuclear @placeholder thinks it can do without facing a broader escalation of tensions."