North and South Korea exchanged fire during military drills near a disputed area in the Yellow Sea. 

White House Press Secretary Jay Carney said, “North Korea’s actions are dangerous and they’re provocative. And the DPRK’s continued threats and provocations aggravate tensions and further North Korea’s isolation.”

He said the U.S. government remains steadfast in its commitment to the defense our allies and is in close communication with the Republic of Korea and Japan.

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South Korean Marines fired artillery shells across a disputed sea border after North Korean shells from a live fire drill conducted by Pyongyang fell into the water south of the frontier, Seoul officials told the Associated Press. 

There were no reports of any injuries. Residents on front-line South Korean islands spent several hours in shelters during the firing, and officials temporarily halted ferry service linking the islands to the mainland. Kang Myeong-sung, speaking from a shelter on Yeonpyeong island, which is in sight of North Korean territory, said he didn't hear any fighter jets but heard the boom of artillery fire.

North Korea fired 500 rounds of artillery shells over more than three hours, about 100 of which fell south of the sea boundary, South Korean Defense Ministry spokesman Kim Min-seok said later Monday. South Korea responded by firing 300 shells into North Korean waters, he said.

No shells from either side were fired at any land or military installations, but Kim called the North's artillery firing a provocation aimed at testing Seoul's security posture. There was no immediate comment from North Korea.

The exchange of fire followed Pyongyang's earlier, unusual announcement that it would conduct the drills, a move seen as an expression of Pyongyang's frustration at making little progress in its recent push to win outside aid.