South Korea ? North Korea declared it would cut all ties with South Korea in response to its blaming of the communist country for the deadly sinking of a South Korean warship, as tensions on the divided peninsula spiked to their highest level in a decade.

The dramatic deterioration in ties came as U.S. Secretary of State Hillary Rodham Clinton visited Seoul today for talks expected to focus on how to deal with North Korea.

The North’s announcement late Tuesday came hours after South Korea began taking steps that were seen as among the strongest it could implement short of military action — ranging from slashing trade, resuming propaganda warfare and barring the North’s cargo ships.

Tensions have risen since last week, when a team of international investigators concluded that a torpedo from a North Korean submarine tore apart the Cheonan warship off the west coast on March 26, killing 46 South Korean sailors.

The North flatly denies involvement in the sinking of the Cheonan, one of the South’s worst military disasters since the Korean War, and has warned that retaliation would mean war.