South Korea's military said on Saturday observers heard an explosive sound near the country's northernmost frontline island of Baengnyeong, adding it possibly came from a Northern military drill.

   "We detected a sound which is presumed to be from the North's coastal artillery around 7:20 a.m.," a military official said, adding the police also made an urgent announcement to the islands' residents a few minutes later.  

   South Korean fishing boats were also requested to return to their ports following the announcement made at 7:47 a.m.

   The firing did not cross the maritime border in the Yellow Sea, widely known as the Northern Limit Line, it also said.

   The military said there have not been further signs of drills detected in the area, adding it will continue to more closely observe the North.

   In November 2010, North Korea fired some 170 rounds of artillery at another frontline island of Yeonpyeong, killing two South Korean marines and two civilians and wounding 16 others.

   Pyongyang's provocations have continued since early 2016.   

   North Korea conducted what it claimed to be a successful test of an H-bomb on Jan. 6. Earlier this month, the North also launched a rocket carrying the Kwangmyongsong-4 satellite, which was viewed by many as a cover for testing its intercontinental ballistic missile technology.

   South Korea closed down the inter-Korean industrial zone in the North Korean border city of Kaesong earlier this month in response to the North's moves. (Yonhap)