"What is obvious so far is that the Cheonan did not sink due to a simple accident," Lee Myung-bak said at a meeting of senior military commanders in Seoul, where he ordered a "full-fledged" review of his country's military readiness.
'International and inter-Korean'
"As soon as the incident occurred, I sensed it was a grave international and inter-Korean matter," he said.
"As soon as the incident occurred, I sensed it was a grave international and inter-Korean matter"
South Korea's president
"After we find the cause, I will take definite, stern" action against those responsible, he said in the nationally-televised speech.
Seoul has never directly blamed North Korea for the sinking, and Pyongyang has denied involvement. However, suspicion has focused on the North.
Kim's previous trips to China have led to steps that have reduced security concerns between the rival Koreas.
In 2000, Kim's trip was soon followed by a summit in Pyongyang with South Korea's leader and the start of two major joint development projects in North Korea.
Analysts say Beijing wants to prevent an escalation of military tension but is unlikely to punish its neighbour even if it were found to be behind the attack on the South Korean warship.
Al Jazeera's Divya Gopalan, reporting from Dalian, said South Korea has expressed a lot of anger over recent incidents and relations with the North continue to deteriorate.
Kim's visit reflects China's stance as North Korea's only ally, our correspondent said.
|A South Korean naval ship sinking killed 46 sailors in March [EPA]
North Korea's economy has been hit by sanctions and Kim needs to talk to the Chinese president, Hu Jintao, to get some support, when the rest of the world has, in effect, turned their backs on the North, she said.
"China also seems to be the only nation pushing North Korea to go back to six-party talks over its nuclear programme," she added.
The North has come under pressure to return to six-country nuclear talks due to UN sanctions imposed after a May 2009 atomic test that have dealt another blow to its already shaky economy.
A 2004 trip to China led to a push for talks on the North's nuclear programmes.
The North Korean leader reportedly entered China by train on Monday and spent the night in Dalian, a thriving city that has attracted major foreign investment and a symbol of development that Beijing's leaders have advocated for years to Kim and his father and state founder Kim Il-sung, to revive the North's moribund economy.
Zhang Liangui, an expert on North Korea at the Central Party School in Beijing, told the Reuters news agency that there would be "discussion of North Korea demands for aid, for food and oil, as well as returning to the nuclear talks".
"But I think the North Korean leader will be most concerned about economic relations, because the domestic economy there is in trouble," he said.
China is a crucial economic and political ally of its much smaller neighbour.
In 2009, bilateral trade between China and North Korea, with an estimated GDP of $17bn, was worth $2.7bn.
Kim is even more reliant on China's help now, after a botched currency reform at the end of last year worsened inflation and sparked rare civil unrest that raised questions about Kim's grip on power in the state his family has run for more than 60 years.