Amid tension, U.S. and South Korea begin military drills

Seoul, South Korea (CNN) -- South Korea and the United States started joint military exercises Sunday, U.S. Forces Korea spokesman David Oten told CNN, prompting a furious response from North Korea.

The naval operations are "no more than an attempt to find a pretext for aggression and ignite a war at any cost," North Korea's official Korean Central News Agency said, warning that the drills "are putting the Korean Peninsula at a state of ultra-emergency."

North Korea warned of unpredictable "consequences" if the United States sends an aircraft carrier to the Yellow Sea for the military maneuvers.

The divided peninsula, tense at the best of times, has been near the boiling point since Tuesday, when North Korea shelled a South Korean island, killing four people.