South Korea fired "tens of shells" across the heavily fortified border with North Korea after a rocket launched from the north landed in Yeoncheon County, South Korea's Defense Ministry said Thursday.
No deaths or injuries were reported. Some residents of nearby villages were evacuated, South Korean news reports said.
The exchange occurred across the demilitarized zone, a 160-mile-long no man's land that has divided the two nations since an armistice ended combat in the Korean War in 1953.
"Our military has increased monitoring and is closely watching North Korean military movement," the ministry said in a statement.
Tension between the two countries has been high since early this month, when Seoul accused North Korean soldiers of sneaking across the zone and planting three land mines near a South Korean military post. Two South Korean staff sergeants were maimed after the mines detonated.
North Korea called the accusation "nonsense" and urged Seoul to back up its allegations with video evidence.
In retaliation, the South blared criticism of the North from loudspeakers along the border that had lain dormant for 11 years. The North attempted to drown out the noise by blasting propaganda from its own loudspeakers.
North Korea on Thursday threatened military action if the South's loudspeaker broadcasts were not halted, the Defense Ministry said in a subsequent statement. The North also said it would not apologize for the recent mine explosion, the ministry said.
The North Korean government in Pyongyang has warned that it would take "all-out military action of justice" against a joint U.S.-South Korean military drill that began Monday. The annual drill — called the Ulchi Freedom exercise — involves tens of thousands of troops.
In October, the two countries exchanged gunfire after South Korean activists sent balloons filled with propaganda leaflets over the border and North Korea attempted to shoot them down. Activists regularly attempt to send message-bearing balloons into North Korea in an effort to expose the country's people to information from the outside world.
Times staff writer Kaiman reported from Beijing, and special correspondent Borowiec from Seoul.