WASHINGTON (AP) _ The United States has been keeping tabs on the contested region of the Yellow Sea where North and South Korean warships exchanged gunfire Tuesday.

Both Koreas claim the area, a rich crab fishing ground, as territorial waters.

North Korea has sent fishing boats and naval ships into the zone 20 to 30 times a year. However, it usually withdraws upon a challenge by South Korean patrol boats.

``From my understanding, this is an annual situation,'' said P.J. Crowley, a spokesman for the National Security Council. ``North Korea has, in the past, attempted to bring vessels down to these waters. In the past, when they've been warned off by South Korean vessels, they've turned around and returned to North Korea. This year, for some reason, they have not.''

Crowley said the Clinton administration is ``very concerned'' about Monday's action by North Korea and is closely monitoring developments.

``There are military-to-military talks going on today to discuss this situation and we are in close touch with the South Korean government regarding the steps they are taking in response,'' he said.

The Korean peninsula was divided into communist North Korea and capitalist South Korea in 1945. They are technically still at war as their 1950-53 Korean war ended with an armistice, not a peace treaty. The armistice, signed by the U.N. Command and North Korea, never outlined the border in the Yellow Sea off the western coast.