North Korea is believed to have stepped up activities at the Punggye-ri nuclear test site ahead of US President Barack Obama's two-day visit to South Korea from 26 April.
"The Punggye-ri nuclear test site has shown increased movement of vehicles and forces compared to the past. South Korean and US forces have been closely monitoring the latest development to detect signs of another test," a senior South Korean military official told Yonhap news agency.
North Korea had earlier threatened a "new form" of nuclear test in response to the US-South Korea joint military drill, which has now ended.
Earlier reports suggested that North Korea was ready to conduct a nuclear test barring final preparations. The country is prohibited from conducting such tests under UN sanctions.
Meanwhile, North Korea's foreign ministry has heaped scorn on Obama's upcoming Asia tour.
The ministry said Obama's trip would "escalate confrontation and bring the dark clouds of a nuclear arms race".
During his week-long tour, Obama will visit four Asian countries – Japan, South Korea, Malaysia, and the Philippines – in Washington's latest bid to reaffirm its commitment towards its allies in the region.