There has been growing speculation that North Korea was behind the March 26 explosion that split the 1,200-ton Cheonan in two and sank it, killing at least 38 people and leaving eight missing.
Seoul has not directly blamed Pyongyang for the blast, and the North has denied involvement, but suspicion remains given the country's history of provocation and attacks on the South.
On Thursday, the mass-circulation Chosun Ilbo newspaper reported the Korea Defense Intelligence Command had alerted the navy weeks ahead of the ship sinking that North Korea was preparing underwater suicide teams in mini-submarines to attack the South.
These "human torpedo" squads were said to involve small submarines that are navigated so close to the target that their torpedoes or explosives blow up both target and the attackers, or are timed to explode while the attackers escape from the vehicle, the report said.
The attack preparations were aimed at retaliating against the South over its defeat in a naval skirmish in November, the paper said. The site of the sinking is near where the rival Koreas fought three times since 1999, most recently a November clash that left one North Korean soldier dead and three others wounded.
South Korea is investigating the and investigators say preliminary indications are that the blast was external, not on board the ship. Some experts say the investigation could take several years.
The two Koreas have never signed a peace treaty since the 1950-53 Korean War ended in a truce.
The Chosun Ilbo said the military was investigating whether the navy and Joint Chiefs of Staff had been properly braced for a North Korean attack following the intelligence warning, though it's not clear whether the Cheonan sank due to any attack.
Navy and Joint Chiefs of Staff officials said they would not comment on the report because it involves military intelligence affairs.
Also Thursday, Yonhap news agency reported that military intelligence officers believe a North Korean submarine torpedoed the Cheonan based on a joint intelligence analysis with the U.S. military.
Yonhap, citing an unidentified military source, said the assessment was submitted to the presidential Blue House shortly after the warship sank. The Blue House, however, denied it has received such an intelligence report.
A high-profile North Korean defector living in Seoul said he believes North Korean leader Kim Jong Il masterminded the blast to stoke tension, cause social confusion in the South and shake its economy.
"It's obvious it's something that Kim Jong Il did. We already know Kim Jong Il has been preparing for this kind of incident," said Hwang Jang-yop, a former secretary of the North's Workers Party who once mentored Kim before defecting to Seoul in 1997, in an interview with Chosun Ilbo published Thursday. He didn't provide any evidence for his belief.
A Seoul-based activist said Thursday that he also believes Kim is behind the attack, citing an unidentified North Korean military officer as saying the leader visited a west coast naval command in February and called for revenge for the earlier defeat. Choi Sung-yong told reporters that the officer told him that a North Korean semi-submersible vessel torpedoed the Cheonan.
Two North Korean agents were arrested Tuesday for allegedly plotting to assassinate the 87-year-old Hwang who has openly condemned Kim's regime as totalitarian. Hwang has shrugged off the plot, saying he wasn't intimidated by it.
Seoul officials have said there has been no definitive evidence indicating the North's involvement yet. President Lee Myung-bak said earlier this week that South Korea would deal "resolutely and unwaveringly" with the outcome of an investigation into the sinking.
Yonhap said South Korea is considering deploying more artillery and anti-missile radars on islands near the western sea border in the wake of the ship sinking. The Joint Chiefs of Staff said it couldn't confirm the report.