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

A defense ministry official in Seoul said all shells fell into the sea and there were no reports of damage or injuries on Yeonpyeong island, where a North Korean artillery attack killed four South Koreans in November. South Korea's Yonhap news agency quoted officials as saying a second short North Korean volley was heard on Yeonpyeong later Wednesday, but it was not clear whether the island was targeted.

South Korean military officials said there were no signs of unusual North Korean troop movements Wednesday, and said southern forces were maintaining a defensive posture.

The South Korean defense ministry said the original shelling apparently came from an island about 11 kilometers north of the border. South Korean troops saw one shell land in the water near Yeonpyeong.

The spokesman said Seoul responded to the first shelling by sending a radio warning to the North then, 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.