The scent of incense filled the air on Monday as the Republic of Korea's president bowed before a memorial for young sailors killed when their warship sank last month in an explosion the defense minister has blamed on a torpedo.

A man grieves during a memorial service for the deceased sailors from the lost ROK naval ship Cheonan on Monday. [Ahn Young-joon / Associated Press]

A solemn President Lee Myung-bak joined mourners in paying his respects to the 46 men who went down with the Cheonan on March 26 after an explosion ripped the ship in two. He laid a single white chrysanthemum at an altar set up for five days of mourning.

"The Republic of Korea will never forget your honorable sacrifice," Lee wrote in a condolence book.

The government has not blamed the Democratic People's Republic of Korea (DPRK) outright for the disaster, one of the ROK's worst.

Pyongyang has denied involvement.

On Sunday, Defense Minister Kim Tae-young said a torpedo was the most likely culprit. Investigators examining the wreckage announced separately that a close-range, external explosion likely sank the 1,200-ton ship.

Kim did not speculate on who may have fired the torpedo, and said it was still too early to determine the exact cause.

The Cheonan was on a routine patrol when it sank in the rough Yellow Sea, not far from the spot where the two Koreas' militaries have clashed three times since 1999, most recently last November.

The "bubble jet effect" from a torpedo - the rapidly expanding bubble an underwater torpedo blast can create and the subsequent destructive column of water unleashed - was the most likely cause, the defense minister told reporters.

Fifty-eight sailors were rescued. Forty bodies have been recovered, and six missing crewmembers are presumed dead, officials said.

Some 7,000 people have visited an altar in downtown Seoul to pay their respects, from war veterans dressed in medal-bedecked uniforms to mothers explaining to their children about the "uncles" who died protecting the country.

One woman sobbed as she stroked a photo of one of the sailors, most of whom were in their 20s.

"We will never forget you," one note on a message board read. "There, in heaven, I hope you will get to live the life you weren't able to live in this world," read another.

"I feel like it was my friends who died on that ship," said college student Chung Jae-mi, 21. "It hurts me to think that they were as old as I am."