South Korea said it launched artillery exercises at 27 points along the coast, though not near the disputed border, The Washington Post reported.
A nationwide civil defense drill is scheduled Wednesday, with citizens taking underground shelter and jets simulating a North Korean air strike.
"For South Koreans, this is like sleeping and dining with an enemy," said Song Min-soon, a former foreign minister. "It's like an invader who has a grenade in his hand with the pin pulled off."
North Korea's main Communist party newspaper, the Rodong Sinmun, carried a commentary Monday that condemned U.S.-South Korea military cooperation as "nothing but treachery escalating the tension between the North and the South and bringing the dark clouds of a nuclear war to hang over the Korean Peninsula."
Such rhetoric is not new, but North Korea's Nov. 23 attack on a South Korean island has given it new force.