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."

"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."

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