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North and South Korean artillery batteries exchanged hundreds of shells across their western sea border Monday, a day after North Korea warned it was preparing to test another nuclear device, CNN reported.

About 100 of the 500 shells North Korea fired into the Yellow Sea strayed across the line separating the two rivals' territorial waters, the semiofficial South Korean news agency Yonhap reported. Yonhap quoted the South Korean Joint Chiefs of Staff as saying the South responded by firing about 300 shells into North Korean waters and dispatching fighter jets to the boundary, known as the Northern Limit Line.

North Korean offshore firing appeared to have resumed after a lull, Yonhap reported, citing a resident of Baekryong Island, which is close to the Northern Limit Line.

"Some (North Korean) artillery fire landed in (the) southern part of Northern Limit Line but in the water," a South Korean Ministry of Defense spokesman said. "We counter-fired over the Northern Limit Line."

When asked what South Korea fired back at, the defense spokesman said, "We are not shooting at North Korea, just shooting into the sea."

The United States, South Korea's leading ally, condemned the North Korean shelling from the White House and the Pentagon.

Washington is working "in close coordination" with South Korea and Japan, White House spokesman Jay Carney said, calling on North Korea "to refrain from actions that threaten regional peace and security."

And Defense Secretary Chuck Hagel told reporters at the Pentagon, "The provocation that the North Koreans have, once again, engaged in is dangerous, and it needs to stop."

China, the North's main patron, also expressed concern.

"The temperature is rising at present on the Korean Peninsula, and this worries us," Foreign Ministry spokesman Hong Lei told reporters in Beijing. "We hope that all sides can remain calm and exercise restraint."