"North Korea will make a decision on conducting a nuclear test based on its political judgment," the South Korean official told Yonhap News Agency.
Speaking on condition of anonymity, the official said major factors in a decision on whether to conduct the test would include "international pressure and sanctions as well as opposition from China and Russia."
The official's comments came a day after North Korea said it would strengthen its nuclear deterrent in the face of pressure from the United States, South Korea and Japan to try to persuade the North not to conduct a nuclear test.
In a report by the official KNCA news agency Tuesday, North Korea's foreign ministry spokesman said the North would take "countermeasures for self-defense" if the United States and South Korea continue to exert pressure on Pyongyang.
The official said South Korean military authorities have detected what Yonhap called "brisk activities" at North Korea's northeastern nuclear test site Punggye-ri, including new roads and structures, mining carts and excavation vehicles.
South Korean and U.S. intelligence authorities are closely watching activity at the test site, the official said.
North Korea launched a long-range rocket April 13 despite international warnings against doing so but the launch failed.
The North conducted nuclear tests after rocket launches in 2006 and 2009.