(CNN) -- The South Korean military said it heard the sound of artillery fire coming from North Korea on Wednesday, but the shells landed on the North's side of the border in the Yellow Sea.

"We have confirmed that North Korean artillery shells have landed in waters adjacent to the North Korean coast," said an official at the South Korean Joint Chiefs of Staff, who is not authorized to be quoted by name. "We are analyzing the origins of this."

The South Korean Yonhap News Agency quoted an official from the South's Joint Chiefs of Staff as saying the firing "is not a situation that raises concerns."

"We assumed North Korea was carrying out its regular firing exercises," Yonhap quoted the official as saying. "The artillery fire was heard coming from a far distance from the maritime border."

The official also said the artillery fire was apparently aimed at North Korea's own waters, not toward the South's islands, Yonhap said.

Tensions mounted between the Koreas on November 23, when North Korea shelled South Korea's Yeonpyeong Island. The attacked killed two marines and two civilians and injured 18 people. The North has accused the South of provoking the attack because shells from a South Korean military drill landed in the North's waters.

CNN's Jiyeon Lee contributed to this report.