South Korean officials have confirmed that the Seoul government has been informed by the United States that radioactivity was detected in North Korea.

The finding suggests the communist state did actually conduct a nuclear test.

An intelligence official in Seoul, who wanted to remain unidentified, said Saturday that the US authorities informed Seoul of the finding. But he said that Washington did not give additional details, including the amount of radioactivity detected and the method used to detect it.

Earlier in the day, CNN reported that the U.S. has preliminary evidence of radioactivity from North Korea's nuclear testing ground, indicating that it did indeed carry out a test.

Reuters also quoted an unnamed U.S. official as saying that a preliminary U.S. intelligence analysis has shown radioactivity in air samples collected near a suspected North Korean nuclear test site.

North Korea claimed Monday that it performed a successful nuclear test. The South Korean authorities immediately confirmed that there was a seismic tremor caused by an "artificial earthquake."