South Korean officials say North and South Korea exchanged artillery fire Wednesday in an area near their tense maritime border in the Yellow Sea.

A defense ministry said all the shots fell into the sea and there were no reports of damage or injuries. The exchange took place near Yeonpyeong island, where a North Korean artillery attack killed four South Koreans in November.

The spokesman said South Korean forces heard several shots fired from North Korea, apparently from an island about 11 kilometers north of the border. The troops saw one shell land in the water near Yeonpyeong.

The spokesman said South Korea responded by sending a radio warning to the North. About 35 minutes later, the South fired three howitzer shells into the sea.

Forces in the area have been on heightened alert since the November attack on Yeonpyeong, a small island hugging the maritime border. The North fired more than 100 artillery shells onto the island, killing two marines and two civilians.

The attack severely damaged relations between the two Koreas, which were already strained over the sinking of a South Korean warship in March 2010.

In the first significant sign of a thaw, senior diplomats held what they described as constructive talks last month on the sidelines of a regional security conference in Bali, Indonesia.

A North Korean delegation later visited New York for additional talks with U.S. officials.