The US is to hold anti-submarine and other naval exercises with Seoul in the "near future" as South Korea halts trade with the North in retaliation for the sinking of one of its warships.
The announcements came after an international investigation last week concluded that a North Korean submarine fired a heavy torpedo at the Cheonan on March 26, sinking the South Korean vessel and killing 46 sailors. North Korea denied the attack.
South Korean President Lee Myung-Bak also banned the North's merchant ships from South Korean waters and said Seoul would refer the attack to the United Nations security council for punishment.
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In a nationally televised address on Monday, a sombre-looking Lee vowed an immediate military response to any future aggression, saying South Korea had in the past repeatedly tolerated the North's "brutality".
"But now things are different. North Korea will pay a price corresponding to its provocative acts," he said, demanding an apology for the sinking of the Cheonan, a 1200-tonne corvette.
"From now on, (South) Korea will not tolerate any provocative act by the North and will maintain the principle of proactive deterrence," Lee said.
"If our territorial waters, airspace or territory are violated, we will immediately exercise our right of self-defence."
Seoul's Defence Minister Kim Tae-Young said the US and South Korean navies would soon stage an anti-submarine drill off the west coast and his government would resume the border broadcasts halted six years ago.
Pentagon touts drills in near future
Pentagon spokesman Bryan Whitman told reporters in Washington the joint anti-submarine and other maritime interdiction exercises would be held in the "near future".
Whitman said the maritime interdiction exercises would be conducted under the US-led Proliferation Security Initiative, designed to stop trafficking in weapons of mass destruction.
US President Barack Obama directed his administration to review North Korea policy and commanded his armed forces to work closely with South Korea "to ensure readiness and to deter future
aggression", the White House said.
Analysts predicted cross-border tensions would remain high for months but said major conflict was unlikely.
A multinational investigation team on Thursday last week reported finding "overwhelming evidence" that a North Korean submarine fired a heavy torpedo which sank the Cheonan near the disputed border.
The United States and numerous other nations have condemned the attack, seen in Seoul as one of the worst provocations since the 1950-53 Korean War. The North's ally China, which wields a Security Council veto, again called on all sides to show restraint.
US Secretary of State Hillary Clinton, during her current visit to Beijing, is pressing China to get tough with its wayward ally. She goes on to Seoul on Wednesday.
In Tokyo, Japan said it supported Seoul's push for security council punishment and was studying more sanctions of its own against Pyongyang.
France said it would work at the security council to ensure North Korea "doesn't go unpunished" for the sinking, characterising it as "criminal aggression".
