North Korea has intensified warnings in recent weeks, declaring it had entered a state of war with Seoul, threatening to strike U.S. targets and blocking access to a border factory complex jointly run with the South.
"There are recent active movements of manpower and vehicles at the southern tunnel at Punggye-ri," South Korea's JoongAng Ilbo newspaper quoted an unidentified government official as saying. The official was referring to North Korea's nuclear test site.
"We are monitoring because the situation is similar to behaviour seen prior to the third nuclear test," the official was quoted as saying. It was unclear, the official told the newspaper, whether the activities were intended to mislead U.S. surveillance.
The North's February 12 nuclear test prompted tougher U.N. sanctions and triggered a hostile response from Pyongyang.
South Korea's defence minister told lawmakers in February, after the third nuclear test, that an additional test was possible.
Pyongyang moved what appeared to be a mid-range Musudan missile to its east coast, according to media reports last week.