Initial inspections of the stern, salvaged from the seabed on April 15, and the bow, which was raised on Saturday, indicate it was hit by the force of a blast, officials said.
Yoon Duk-Yong, co-chairman of a joint international investigation team including US and Australian experts, issued an interim report that no soot, melting or any explosion holes were found on the ship.
"In conclusion, after the initial visual inspection of the severed surface and the inside/outside of the hull, we assume the cause is underwater explosion," he said.
|Up to 40 sailors died and six went missing when the Cheonan capsized on March 26 [Reuters]
"And looking at the form of the deformation, it is highly likely that a non-contact explosion was the cause rather than a contact explosion."
Yoon did not specify what type of explosive could have been detonated.
Shortly after the vessel sank, Kim said a mine or a torpedo could have been to blame, but since then the government has been careful to avoid pointing a finger at the North.
In recent days several unidentified sources were quoted as saying an underwater explosion was the cause of the sinking.
However, Kim is the first minister to make the link explicitly since an investigation into the sinking began.
Chung Un-Chan, South Korea's prime minister, declared a five-day "national mourning period" for the 46 sailors from Sunday until Thursday, with public shrines set up in Seoul and other cities nationwide for citizens to pay tribute.
Pyongyang has accused the South's "war maniacs" of seeking to shift the blame for the tragedy to the North.
The North seized South Korean-owned assets at a mountain resort on Friday, and said that the two countries were on the brink of war over the sinking.
The next day, North Korea said it would use "all means, including the nuclear deterrent" if it was invaded by the US and South Korea.
Lee Myung-Bak, South Korea's president, vowed on Wednesday a "resolute" response to the Cheonan disaster, calling the worst peacetime loss of life for South Korea's navy a "wake-up call" and describing the North as the world's "most belligerent" state.
The disputed Yellow Sea border was the scene of deadly naval clashes between the North and South in 1999 and 2002 and of a firefight last November that left a North Korean patrol boat in flames.