North Korea has accused South Korea of illegally using its television and radio frequency to air propaganda broadcasts.
A spokesman at the Ministry of Posts and Telecommunications said Saturday that the South Korean broadcast is a grave challenge because it tries to slander the North's leadership and incite anti-government sentiment.
The warning, carried by the official Korean Central News Agency, says Seoul won't avoid "deadly" and "merciless" retaliation if it doesn't stop its anti-Pyongyang broadcasts.
North Korea has issued a series of similar unspecified threats against the government of President Lee Myung-bak.
Seoul's Defense Ministry says it has no immediate comment.