NBC News, meanwhile, reported that an increase in radioactivity has been observed around areas where North Korea's nuclear test sites appear to be located as a result of a U.S. analysis of air over the areas.

A further analysis is necessary to determine that a nuclear test has been performed there as claimed by North Korea, the CNN report said.

Earlier in the day, CNN reported, citing two U.S. government officials with access to classified information, that the initial air sampling over North Korea shows no indication of radioactive debris from the event Monday that North Korea says was an underground nuclear test.

The U.S. Air Force flew a WC-135 Constant Phoenix on Tuesday to collect air samples from the region, the report said, adding a third official reiterated that at this point "there isn't information to allow confirmation it was a nuclear test."

The intelligence community and the military will continue to fly satellites and collect air samples in the region to try to collect radiological data that would provide confirmation of a nuclear test, officials were quoted as saying. But as time goes on, it will be increasingly difficult to achieve confirmation.

Officials emphasize this is preliminary data, and it provides no conclusive evidence about the North Korean event, the report said.