SEOUL, South Korea — South Korea blared propaganda broadcasts into North Korea today after a six-year halt and Pyongyang said its troops were bracing for war as tensions spiked on the divided peninsula over the sinking of a warship.

One Seoul-based monitoring agency reported that North Korea's leader ordered its 1.2 million-member military to get ready for combat after South Korea blamed the North for a March 26 torpedo strike that sank the warship Cheonan and killed 46 sailors. South Korean officials could not immediately confirm the report.

The South's restarting of psychological warfare operations — including radio broadcasts into the North and placing loudspeakers at the border to blast out propaganda — were among measures the government announced Monday to punish Pyongyang. The South is also slashing trade and denying permission to North Korean cargo ships to pass through South Korean waters.

A team of international investigators concluded last week that a torpedo from a North Korean submarine tore apart the Cheonan. The sinking was one of the South's worst military disasters since the 1950-53 Korean War, which ended with an armistice, not a peace treaty.

The North flatly denies involvement and has warned such retaliation would mean war. It has threatened to destroy any propaganda facilities installed at the heavily militarized border.

Today, the North's military claimed dozens of South Korean navy ships violated the countries' disputed western sea border earlier this month and threatened to take "practical" military measures in response.

South Korea's military had no immediate response other than to say that North Korea routinely makes similar accusations.

North Korea is already subject to various U.N.-backed sanctions following earlier nuclear and missile tests. The latest steps announced by Seoul were seen as among the strongest it could take short of military action.

The U.S. has thrown its full support behind South Korea's moves and they are planning two major military exercises off the Korean peninsula in a display of force intended to deter future aggression by North Korea, the White House said. The U.S. has 28,500 troops in South Korea.

South Korea also wants to bring North Korea before the U.N. Security Council over the sinking. U.N. Secretary-General Ban Ki-moon said Monday he expects the council to take action against North Korea, but China — North Korea's main ally and a veto-wielding council member — has so far done little but urge calm.

U.S. Secretary of State Hillary Rodham Clinton said she had "very productive" discussions with Chinese officials but could not say if any progress had been made in convincing the Chinese to back U.N. action.