Lee Myung-bak praised the sailors for their "lofty sacrifice"
The South Korean president has marked the "lofty sacrifice" of 46 South Korean sailors who died when a warship mysteriously sank last month.
Lee Myung-bak was one of many mourners to pay last respects to the sailors at altars set up across the country.
Officials say the sinking of the Cheonan was due to a "close-range" blast which split the ship in two.
Local analysts say this could have been caused by a torpedo or a mine exploding below the 1,200-tonne corvette.
Fifty-eight sailors were rescued after the explosion on 26 March.
The incident has fuelled tensions with North Korea, which many South Koreans believe sank the ship.
The government in Seoul has so far taken care not to directly accuse Pyongyang, which denies any responsibility.
However, South Korea is now reportedly investigating whether a demoted North Korean general has won back his former rank as a reward for the sinking the vessel.
Seoul has also launched an investigation into its own response to the disaster, after questions were raised about the speed of the rescue operation, South Korean news agency Yonhap reported.
Mr Lee visited one of the altars on Monday, a day after South Korea began five days of national mourning for the victims.
The Cheonan was split in half and sunk by an unexplained explosion
"The Republic of Korea will not forget your lofty sacrifices," he wrote in a condolence book at the shrine, according to reports by South Korean media.
Mr Lee said public officials "should keep the sailors in mind" during the official mourning period, indicating they should avoid entertainment activities.
The mourning will culminate in a mass funeral on Thursday for all 46 sailors, including the six whose bodies have not been found but whose families have asked that they be considered killed in action.
After examining the bow of the warship, salvaged last Saturday, officials said the vessel was damaged by a "bubble jet" caused by an external underwater blast.
"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," an official quoted by Yonhap said.
South Korean Defence Minister Kim Tae-young said that although the bubble jet effect was the most likely explanation "the possibility of other causes is still under investigation".
The minister said it would take about a month for the investigation team to come up with the final results of its inquiry.
An earlier investigation has already concluded that the explosion which sank the ship was external, fuelling suspicions of North Korean involvement.
The two countries are still technically at war since the 1950-53 war ended without a peace treaty. There have been several naval clashes in the area that the Cheonan went down, off the west coast of the peninsula.
Meanwhile, South Korean officials have said a "full-scale audit" of the official response to the disaster will be launched immediately after the sailors' funeral.
An official at the Board of Audit and Inspection (BAI) told Yonhap they were currently collecting evidence from the Defense Ministry and the Joint Chiefs of Staff to address allegations that the sinking was not immediately reported to Mr Lee and that it took too long to locate the sunken vessel.
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