`` The U.S. had better ponder over this and stop acting rashly . '' North Korean military exercises see shelling into South Korean waters ; South responds with artillery fire . - The United States and Japan hold a joint exercise in the Sea of Japan. North Korea hints at further nuclear tests . Claim comes days after North 's test-firing of medium-range missiles into the sea .
North Korean military exercises see shelling into South Korean waters ; South responds with artillery fire . - The United States and Japan hold a joint exercise in the Sea of Japan. North Korea hints at further nuclear tests . Claim comes days after North 's test-firing of medium-range missiles into the sea .