(CNN) -- U.S. President Barack Obama on Monday directed military commanders to work with South Korean troops "to ensure readiness and to deter future aggression" from North Korea.
The White House statement came several hours after South Korean President Lee Myung-bak announced that his country was suspending trade with North Korea, closing its waters to the North's ships and adopting a newly-aggressive military posture toward its neighbor in the wake of the sinking of a South Korean warship in March.
South Korean military officials Thursday announced the results of an official investigation into the sinking of the ship, the Cheonan, which concluded that North Korea fired a torpedo that cut the vessel in half.
"We endorse President Lee's demand that North Korea immediately apologize and punish those responsible for the attack, and, most importantly, stop its belligerent and threatening behavior," the statement said.
North Korea has denied that it sunk the warship, which went down on March 26, killing 46 sailors.
"We have always tolerated North Korea's brutality, time and again," Lee said in a nationally televised speech Monday morning. "We did so because we have always had a genuine longing for peace on the Korean Peninsula. But now things are different."
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"North Korea will pay a price corresponding to its provocative acts," he said, according to an English translation of the speech provided by Lee's office. "I will continue to take stern measures to hold the North accountable."
Lee said his country was adopting a posture of "proactive deterrence" toward the North, announcing that "combat capabilities will be reinforced drastically" and that he will focus on improving national security readiness and military discipline.
"If our territorial waters, airspace or territory are violated, we will immediately exercise our right of self-defense," Lee said.
Addressing the alleged attack, Lee said, "Once again, North Korea violently shattered our peace. The sinking of the Cheonan constitutes a military provocation against the Republic of Korea by North Korea."
Lee also admonished his own people, saying that until the attack "we had been forgetting the reality that the nation faces the most belligerent regime in the world."
South Korea's leader called on the North to apologize for the alleged attack and to punish those responsible.
Lee said the alleged attack violated the armistice and nonaggression agreements between the two countries, and he said he will refer the incident to the U.N. Security Council "so that the international community can join us in holding the North accountable."
The White House said it supported bringing the issue before the council.
Tensions between the nations have escalated since the South released its report on the sinking.
On Monday, North Korea threatened it would fire at South Korean loudspeakers if they resume broadcasting along the heavily armed border between the new nations, South Korea's Yonhap news agency reported.
North Korea threatened Friday to back out of a nonaggression pact with the South after Lee vowed "resolute countermeasures" against the North.
"Firstly, from now on (North Korea) will regard the present situation as the phase of a war ...," the North's Committee for the Peaceful Reunification of Korea said Friday, according to Yonhap.
Should South Korea take steps to retaliate, North Korea will "strongly react to them with such merciless punishment as the total freeze of the inter-Korean relations, the complete abrogation of the north-south agreement on nonaggression and a total halt to the inter-Korean cooperation undertakings," the committee's statement said, Yonhap reported.
Lee's speech on Monday brimmed with references to the long history of acrimony between North and South Korea. He spoke from the War Memorial of Korea, the country's main war history museum, and noted that this year marks the 60th anniversary of the outbreak of the Korean War.
"Permeating this War Memorial of Korea are the spirits of the soldiers of the Republic of Korea and United Nations who shed blood on this land," he said. "Also dedicated here was the monument memorializing the 46 fallen warriors of the corvette Cheonan."
U.S. Secretary of State Hillary Clinton is scheduled to meet with Lee and his senior advisers on Wednesday as part of her weeklong Asia trip.
At a speech Friday in Tokyo, Japan, she sharply condemned the attack.
"I think it's important to send a clear message to North Korea that provocative actions have consequences," she said. "We cannot allow the attack on South Korea to go unanswered by the international community."
