South Korea's Navy and coastguard on Tuesday continued their rescue efforts to search for the missing sailors of a naval vessel that sank on Friday, but have yet to break into the sunken ship.

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.

The Navy on Tuesday employed 154 divers, 16 naval ships, three coastguard vessels and 15 U.S. military divers in the search operation, trying to gain access into the sunken ship's stern where most of the missing sailors were thought to be trapped. Some civilian divers also joined the operation. However, divers failed to enter the wreckage due to strong underwater currents.

Late Monday, divers reached the stern of the 88-meter-long ship and succeeded in injecting oxygen through a crack, as the 69-hour window during which rescuers presumed the missing sailors could survive if trapped inside the sealed cabin of the ship ended on Monday evening.

Military divers planned to drill holes to enter the wreckage, but senior officers said it would take much time to gain access.

According to local media, the rescue operations faced difficulties due to extremely unfavorable meteorological and hydrological conditions. Three South Korean military divers fell unconscious due to the strong underwater pressure and currents, one of whom died after moved to a nearby U.S. rescue ship.

The military released a video image of the frigate Cheonan as it went down on Friday night.

The image, taken by a marine unit who used a thermal observation device shortly after it heard the explosion that caused the sinking of the ship, only showed the bow of the ship remained above the waters, local media quoted Defense Ministry spokesman Won Tae-jae as saying.

But he said the video would not be disclosed to the public because there was no clue in the video to help determine the cause of the incident.

Meanwhile, President Lee Myung-bak paid a visit to the country's northernmost island of Baekryeong, the closest island to the scene of the warship sinking, to inspect the navy's search operation.

Faced with mounting public criticism over the government, which has failed to give a clear explanation as to the cause of the incident, Cheong Wa Dae spokesman Park Sun-kyu said Lee's visit should not be interpreted as a political stunt.

Lee earlier in the day convened a Cabinet meeting, where he urged the authorities to conduct a scientific investigation into the naval incident to deter speculation.

"Since the incident happened at the frontline, the government should be thoroughly prepared to cope with any movement on the part of North Korea (DPRK). The armed forces are urged to maintain full alert without the slightest breach," Cheong Wa Dae said in a statement, suggesting that it does not rule out the possibility of the DPRK's involvement in the tragedy.