North Korea has built a half-size mock-up of the South's presidential palace at a firing range in apparent preparation for a live firing drill, according to the South's military chiefs.
The Joint Chiefs of Staff said the information came from analysis of South Korean satellite imagery.
North Korea has in recent weeks repeatedly threatened to strike the presidential palace and government buildings in Seoul amid annual joint military drills between the US and South Korea and heightened sanctions against the North over its recent nuclear test and long-range rocket launch.
North Korea typically conducts more weapons tests when South Korean and US troops conduct springtime drills that it views as an invasion rehearsal. This year's drills end later this week.
Last week, South Korea's president said there were signs North Korea was preparing for a fifth nuclear bomb test amid media reports of increased activity at the country's main nuclear test site.
North Korea carried out a fourth nuclear test in January.