The skirmish began on Tuesday when the DPRK warned the RoK to halt military drills near their sea border, according to Republic of Korea officials. When Seoul refused and began firing artillery into disputed waters - but away from the North Korean shore - the DPRK retaliated by shelling the small island of Yeonpyeong, which houses Republic of Korea military installations and a small civilian population.

Seoul responded by unleashing its own barrage from K-9 155mm self-propelled howitzers and scrambling fighter jets. Two RoK marines were killed in the shelling that also injured 15 troops and three civilians. Officials in Seoul said there could be considerable DPRK casualties.

President Barack Obama pledged that the U.S. would defend the RoK. Trying to avoid any escalation, the U.S. did not reposition any of its 29,000 troops in the South.

The RoK president convened an emergency security meeting shortly after the initial bombardment while China appealed for both sides to remain calm and  U.N. Secretary-General Ban Ki-moon called for "immediate restraint".

AP