Last week, Lee vowed a "resolute" response to the Cheonan disaster, calling the incident a "wake-up call" and describing North Korea - suspected but not publicly blamed - as the world's "most belligerent" state.
The South Korean government has vowed it will take action against those responsible, and its defence minister has said a torpedo attack is among the "most likely" causes of the sinking.
"A bubble jet caused by a heavy torpedo [attack] is thought to be one of the most likely things to be blamed, but various other possibilities are also under review," Kim Tae-young said on Sunday.
|Kim Myong-guk appears to have gained a rank in the wake of the Cheonan's sinking [AFP]
The vessel was split into two by the explosion.
The disputed Yellow Sea border, where the incident occurred, 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.
South Korea has said it will try to see whether Kim Myong-guk, the North's head of general staff operations bureau, had been rewarded for the Cheonan's sinking.
Kim was demoted to a three-star general in a previous move, but appears to have regained his former four-star rank.
North Korea has accused the South's "war maniacs" of seeking to blame it for the tragedy.
Ties between the two Koreas appeared to have entered a new phase of reconciliation after an historic inter-Korean summit in 2000.
But they have suffered several setbacks since Lee's government took power in 2008.