A joint civilian and military investigation team in South Korea gave its initial findings that a "close-range external explosion" might be the cause of a Navy warship's sinking last month, Seoul's Yonhap News Agency on Sunday quoted government officials as saying.

ROK warship sinking caused by external explosion.

"An initial inspection of the bow of the Cheonan salvaged Saturday led investigators to reach a preliminary conclusion that the warship was destroyed by a non-contact external explosion," a ranking government official told Yonhap.

"Instead of being directly hit by a torpedo or other underwater weapon, the Cheonan was affected by a strong explosion that occurred below its bottom at a close range," the official was quoted as saying.

The 1,200-ton Navy frigate "Cheonan", with 104 crew members onboard, went down on March 26 off the South Korean island of Baekryeongdo off the west coast due to an unexplained explosion that split it in two. Fifty-eight sailors were rescued alive, while 40 sailors were confirm dead. Six others are still unaccounted for but are believed dead.

The bow of the sunken ship was salvaged on Saturday while the stern of it was raised out of water on April 15.

According to local media, the initial investigation conducted after the stern was salvaged showed that the propeller and keel appeared not to have been seriously damaged, however, there is a C- shaped cut in the right side of the stern, which indicates that the vessel "might have suffered a strong external shock".

Meanwhile, South Korean Prime Minister Chung Un-chan announced a five-day nationwide mourning period in tribute to the Cheonan victims from Sunday, during which the government will set up an altar at Seoul Plaza, the central of the capital city to allow citizens to offer their condolences. The government also decided to set April 29 as the national mourning day while hold a Navy funeral for the victims at the same day.