In Washington, the Obama administration said the South Korean measures were “entirely appropriate.”
“U.S. support for South Korea’s defense is unequivocal,” a White House statement said, “and the president has directed his military commanders to coordinate closely with their Republic of Korea counterparts to ensure readiness and to deter future aggression.”
The steps outlined by Mr. Lee in a nationally televised speech — coupled with new moves by South Korea’s military to resume “psychological warfare” propaganda broadcasts at the border after a six-year suspension — amounted to the most serious action the South could take short of an armed retaliation for the sinking of the ship, the South’s worst military loss since the Korean War ended in a truce in 1953.
“We have always tolerated North Korea’s brutality, time and again,” Mr. Lee said. “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.”
North Korea’s military immediately warned that if South Korea put up propaganda loudspeakers and slogans at the border, it would destroy them with artillery shells, the North’s official K.C.N.A. news agency reported.
Mr. Lee’s speech came as Secretary of State Hillary Rodham Clinton was visiting Beijing and pressing China to take a much tougher position toward North Korea, China’s historical ally.
The speech was bound to intensify pressure on the Chinese, who have called for restraint.
North Korea has denied responsibility for the sinking of the South Korean warship, the Cheonan, on March 26, which left 46 sailors dead. A growing body of evidence assembled by the South has suggested a North Korean torpedo sank the ship.
Cutting off trade with North Korea is the most punishing unilateral action the South could 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, which 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.”
In a separate announcement, the Defense Ministry announced the resumption of propaganda blitzes aimed at the North, a cold war tactic with loudspeaker broadcasts along the border, propaganda radio broadcasts and leaflets dropped by balloon. The resumption was bound to irritate the North Korea leader, Kim Jong-il, whose grip on power rests partly on denying outside information to citizens.
North Korea has already warned that such a move would prompt it to shut down the border with the South completely, raising the possibility of stranding 1,000 South Korean workers at a joint industrial park in the North Korean town of Kaesong.
President Lee cited evidence that a multinational team of investigators released last week on the sinking of the ship, saying “no responsible country in the international community will be able to deny the fact that the Cheonan was sunk by North Korea.”
But he did not mention China by name.
Mr. Lee also stopped short of terminating the Kaesong industrial complex.
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.
“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.”