South Korea's Ministry of Defense said on Thursday that no submarine of the Democratic People's Republic of Korea (DPRK) was detected at the time of its warship Cheonan's sinking Friday night.

In a 27-page press release, the ministry said "no signs of North Korean (DPRK) submarine activity were detected in the area at the time of the incident".

The 1,200-ton South Korean naval vessel with 104 crew members onboard sank into waters off the west coast of the Korean Peninsula last Friday night due to an unexplained explosion.

The ministry also said another South Korean naval vessel Sokcho fired at an unspecified target toward the north in response after it found a "fast-moving" target in its radars, believing it was a vessel from the DPRK that launched an attack to Cheonan, but the object was found to be a "flock of birds".

However, the ministry said the military are continuing to monitor the DPRK's movements closely, and pledged to release all of the probe results to the public.

The incident occurred less than three months after tensions abruptly rose on the Korean peninsula following the DPRK's repeated firing of artillery shells earlier this year into waters near the de facto sea border, where the two sides had naval skirmishes before.

Pyongyang refuses to acknowledge the maritime border, which was fixed unilaterally by the U.S.-led United Nations Command after the 1950-1953 Korean War.

It is believed that the incident is one of the biggest disasters in the country's naval history that left a 1,200-ton warship torn apart and submerged, while 46 out of 104 crew members are still missing despite continued search and rescue operations.