TOKYO - North Korea on Sunday threatened to fire cross-border shots if South Korea continues a leaflet-launching propaganda campaign, which aims in part to inform the hermetic North of popular revolts in the Middle East.

In a statement carried by its state-run news agency, Pyongyang called the leaflets - stuffed into columnlike balloons - a psychological plot to "shake up our socialism and break the trust of our military and people." Calling it a matter of self-defense, North Korea said it would "launch direct, targeted firing attacks" on any area where South Korean activists or military members are seen releasing the balloons.

Although the North issues frequent threats to its neighbor, the latest warning comes one day before a U.S.-South Korea military drill that Pyongyang considers a preparation for invasion.

In a separate statement carried by the official Korean Central News Agency, Pyongyang also threatened to bolster its nuclear arsenal in response to the "dangerous" U.S.-South Korea military drills. The North warned that it was willing to start a "full-scale" war if provoked.

Sunday's statements brought an end to North Korea's recent peaceful overtures, which included a New Year's Day newspaper editorial that called for improved inter-Korean relations and eventual reunification. This month, military-level talks between the North and South collapsed as North Korean officials walked out of a meeting at the demilitarized zone. Since then, there has been no push for further diplomacy.

North Korea has a history of reacting harshly to propaganda from the South. Last year, when South Korea said it would resume propaganda broadcasts via loudspeakers along the DMZ, the North threatened to fire at the speakers; South Korea soon backed away from its plans.

This time, however, South Korea's campaign is already underway. A Seoul lawmaker said Friday that the military has recently been dropping leaflets into North Korea that describe the pro-democracy movement that ousted Egyptian President Hosni Mubarak and has led to a bloody standoff in Libya. The balloons also contain DVDs about the protests and other basic supplies - food, radios and medicine.

Experts see little chance that a similar uprising will take place in North Korea, which for decades has kept its citizens obedient through surveillance and threats. Even during a mid-1990s famine that caused roughly 1 million deaths, there were almost no signs of public dissent. Citizens now have nominally greater access to outside information, as smuggled DVDs and media trickle in from China, but those unhappy with their leadership have no freedom to organize or talk to one another.

A book by economists Marcus Noland and Stephan Haggard, released last month and based on surveys of North Korean refugees, describes rising dissatisfaction with Kim Jong Il's leadership but notes that "there are reasons to doubt that a dramatic political unraveling will occur in North Korea anytime soon."