SEOUL - South Korea's Navy fired shots after a shell presumably fired by the Democratic People's Republic of Korea (DPRK) fell near disputed western sea border, the Joint Chiefs of Staff (JCS) said here Wednesday.

The DPRK fired two artillery shots toward the so-called Northern Limit Line (NLL) in the Yellow Sea at around 7:46 pm local time and one of the shots was estimated to have fallen near the disputed sea border, the Joint Chiefs of Staff (JCS) was quoted as saying.

South Korea's Navy responded at around 8:02 pm local time with three warning shots, the JCS added.

It was the second artillery shells fired by the DPRK that fell near the NLL. At around 1:00 pm local time earlier in the day, the DPRK fired three shells, and the South Korea's Navy returned three warning shots one hour later, according to the JCS.

But the JCS said it cannot determine whether the shell actually cross the NLL.

" It is our assessment that it was part of a training exercise by the DPRK," a Seoul military official said.

Explaining why the South took an hour to respond, a defence official said: "We could not get visual confirmation about where the North's shells landed, and we needed to use equipment to assess where they landed."

South Korea is keeping tabs on the DPRK, which might have been engaged in routine drills, according to the JCS.

The DPRK shelled a South Korean island near the disputed sea border last November, killing four South Koreans. Pyongyang said the incident was provoked by a drill of Seoul.