“We have always tolerated North Korea’s brutality, time and again,” Mr. Lee said in a nationally televised speech. “But now things are different. North Korea will pay a price corresponding to its provocative acts.

“Trade and exchanges between South and North Korea will be suspended,” he added.

Cutting off trade with North Korea is probably the strongest unilateral action the South can take against the impoverished North. South Korea imports $230 million worth of seafood and other products from the North a year. North Korea earns $50 million a year making clothes and carrying out other business deals with South Korean companies.

Mr. Lee also said that South Korea would block North Korean merchant ships from using South Korean waters off the southern coast. That would force the ships to detour and use more fuel.

Besides these unilateral measures, South Korea will “refer this matter to the U.N. Security Council, so that the international community can join us in holding the North accountable,” Mr. Lee said. “Many countries around the world have expressed their full support for our position.”

Citing evidence that a multinational team of investigators released last week on the sinking of the ship, the Cheonan, on March 26, Mr. Lee said that “no responsible country in the international community will be able to deny the fact that the Cheonan was sunk by North Korea.”

But he stopped short of mentioning China by name. Any international efforts to punish the North through sanctions can have only a limited impact on the isolated North, unless China, the main trade partner and aid provider for the North, participates. So far, Beijing has been noncommittal, maintaining its stance of urging both Koreas to exercise restraint.

Mr. Lee also stopped short of terminating a joint industrial complex at the North Korean city of Kaesong, where hundreds of South Korean factories hire about 46,000 North Korean workers. The complex remains the most visible symbol of inter-Korean economic exchanges and political reconciliation.

Delivering his speech from the Korean War Memorial in Seoul, Mr. Lee drew an analogy between the North’s surprise invasion that started the three-year Korean War on June 25, 1950, and the blast that sank the Cheonan, which killed 46 sailors.

“Again, the perpetrator was North Korea. Their attack came at a time when the people of the Republic of Korea were enjoying their well-earned rest after a hard day’s work,” he said. “Once again, North Korea violently shattered our peace.”

He urged North Korea to apologize and punish those responsible. North Korea has denied responsibility.