Seoul, South Korea ? Military tension on the Korean peninsula rose Thursday after North Korea threatened to attack any South Korean ships entering its waters and Seoul held anti-submarine drills in response to the March sinking of a navy vessel blamed on Pyongyang.

Separately, the chief U.S. military commander in South Korea criticized the North over the sinking of the South Korean warship Cheonan in which 46 sailors died, telling the communist country to stop its aggressive actions.

North Korean reaction was swift. The military declared it would scrap accords with the South designed to prevent armed clashes at their maritime border, including the cutting of a military hot line, and warned of “prompt physical strikes” if any South Korean ships enter what the North says are its waters in a disputed area off the west coast of the peninsula.

A multinational team of investigators said May 20 that a North Korean torpedo sank the 1,200-ton ship. Seoul announced punitive measures, including slashing trade and resuming anti-Pyongyang propaganda over radio and loudspeakers aimed at the North. North Korea has denied attacking the ship, which sank near disputed western waters where the Koreas have fought three bloody sea battles since 1999.

“The facts and evidence laid out by the joint international investigation team are very compelling. That is why I have asked the Security Council to fulfill their responsibility to keep peace and stability … to take the necessary measures, keeping in mind the gravity of this situation,” U.N. Secretary-General Ban Ki-moon said as he opened a conference in Brazil meant to help find solutions to global conflicts.

Inter-Korean political and economic ties have been steadily deteriorating since the February 2008 inauguration of South Korean President Lee Myung-bak, who vowed a tougher line on the North and its nuclear program. The sinking of the Cheonan has returned military tensions — and the prospect of armed conflict — to the forefront.

Off the west coast, 10 South Korean warships, including a 3,500-ton destroyer, fired artillery and other guns and dropped anti-submarine bombs during a one-day exercise to boost readiness, the navy said.

South Korea also is planning two major military drills with the U.S. by July in a display of force intended to deter aggression by North Korea, according to South Korea’s Joint Chiefs of Staff.

Gen. Walter Sharp, chief of the 28,500 U.S. troops in South Korea, said the United States, South Korea and other members of the U.N. Command “call on North Korea to cease all acts of provocation and to live up with the terms of past agreements, including the armistice agreement.”

The U.S. fought on the South Korean side during the 1950-53 Korean War, which ended in an armistice, not a peace treaty. North Korea has long demanded a permanent peace agreement.