Alleged agent detained on suspicion of plotting to kill high-profile defector Hwang Jang-yop, who died of heart failure.
North Korean troops have fired across the border into South Korean territory, media in the South has reported.
The South Korean military confirmed Friday’s incidents.
The North fired two bullets at a frontline guard post at 5:26pm (08:26 GMT) and South Korean soldiers immediately fired three shots in return from a machine gun, the South Korean Joint Chiefs of Staff said.
“There were no more shots afterwards. We are now closely watching their movements,” a spokesman told the AFP agency.
He said no South Koreans were hurt in the firing near the Demilitarised Zone (DMZ) dividing the two nations.
South Korea media said the shots were fired in Cheorwon in the eastern province of Gangwon.
There were no reports of injuries.
Donald Kirk of the Christian Science Monitor newspaper in Seoul told Al Jazeera that there have been many similar incidents at the border over the years but that the timing of this shooting was surprising.
“It comes as South Korea and North Korea are preparing for a family reunion this weekend. About 100 [South Korean] families are going to North Korea to see their long-lost loved ones,” he said.
“And, South Korea has just shipped 5,000 tonnes of rice to North Korea.”
The incident also comes as the South prepares to host the Group of 20 summit on November 11-12.
South Korea’s military was put on top security alert this week to guard the meeting against any disruptions by North Korea or other elements.
The summit will be attended by world leaders including Barack Obama, the US president, and is being considered the nation’s biggest appearance on the world stage since the 1988 Seoul Olympics.
Cross-border tensions have been high since the South accused the North of torpedoing one of its warships in March, killing 46 sailors. Pyongyang denied any role in the sinking.
Earlier on Friday the North said relations would face a “catastrophic impact” if South Korea persists in rejecting military dialogue aimed at easing tensions on the peninsula.
The first inter-Korean military talks for two years ended without progress in September after Seoul demanded an apology from Pyongyang for the warship sinking.
The two Koreas are technically still at war after signing only a truce to halt hostilities in the 1950-53