“We’re checking the origin of the sounds,” a Joint Chiefs of Staff spokesman told AFP on Friday.
On Tuesday, Seoul and Pyongyang exchanged artillery fire on the Yellow Sea border island of Yeonpyeong and each blamed the other side for starting the skirmish.
South Korea says it returned fire after North Korean forces shelled one of its islands. But the North says the South fired first. Two South Korean Marines were killed in the clashes.
US President Barack Obama has promised “unshakeable support” for South Korea following Seoul’s exchange of artillery fire with North Korea on its western sea border.
In an interview with ABC News on Tuesday, Obama called North Korea “a serious and ongoing threat that needs to be dealt with.”
The US president meanwhile described South Korea as an important ally and “a cornerstone of US security in the Pacific region.”
However, he reportedly ruled out any US military action against North Korea at this stage