President Lee Myung-bak made the remark Sunday at the fifth trilateral summit, hosted by Chinese Premier Wen Jiabao. Japanese Prime Minister Yoshihiko Noda also attended.
Wen said the three countries had the "pressing" task to calm tensions on the Korean peninsula and return to dialogue. China is North Korea's closest ally.
Noda said the three nations should work together to try to prevent further provocations from North Korea.
A failed rocket launch by North Korea last month drew sanctions from the U.N. Security Council, and there are now fears Pyongyang is preparing to conduct its third nuclear test.