North Korea has threatened to strike South Korea "without notice" in response to anti-Pyongyang rallies earlier this week, officials said.
The warning was communicated in a message sent on Thursday by the secretariat of the National Defence Commission, the North's highest military body, through a military hotline, the South's defence ministry said.
The message comes after South Korean conservative groups staged protests against North Korea's human-rights record, marking the two-year anniversary of former leader Kim Jong-il's death by burning his photograph.
North Korea regularly issues idle threats of violence against South Korea and the United States.
In the latest threat, North Korea's military warned of a possible strike and condemned the South Korean rallies as an insult to North Korea's "highest dignity" - a reference to leader Kim Jong-un, the South's Yonhap news agency reported.
In response, the South Korean government vowed to "sternly react" to any provocation, the report noted.
The news comes a week after the North announced it had executed Kim Jong-un's politically powerful uncle, Jang Song-thaek, who was branded a traitor and stripped of all his powers.
Despite the political uncertainty now gripping the country, North Koreans gathered earlier this week for a remembrance ceremony to honour Kim Jong-il.
Top US officials, meanwhile, have speculated that Jang's execution could be a prelude to some kind of provocation by Pyongyang.
"These kind of internal actions by dictators are often a precursor to provocation to distract attention from what they're doing inside of that country," General Martin Dempsey, chairman of the US military's Joint Chiefs of Staff, told a Pentagon news conference on Thursday.
Tensions were heightened earlier this year during US-South Korean military drills that Pyongyang branded a "rehearsal for invasion".