The U.S. government has yet to confirm independently the reports of the nuclear test conducted by the Democratic People's Republic of Korea (DPRK), U.S. State Department spokesman Kurtis Cooper told Xinhua on Sunday.
Meanwhile, U.S. media reports quoted U.S. military officials as saying that they believed the reports on the DPRK's first nuclear test were true, though they were still assessing seismic and intelligence data to make a firm confirmation.
The CNN cited U.S. Defense Department officials as saying that they had no absolute confirmation of the DPRK nuclear test so far, but they had seismic data and other types of intelligence data that they were looking through at this moment.
The assessment might take several hours or more to draw a final conclusion, they said.
According to U.S. media reports, the U.S. government is taking the reports seriously and a wide range of U.S. government agencies are looking into the matter.
There are also reports saying the U.S. Geological Survey has detected "some seismic activity" in the DPRK.
"We did locate some seismic activity. We are working on that event," a Geological Survey official said on Monday.
The DPRK's official Korean Central News Agency reported on Monday that the country successfully conducted an underground nuclear test earlier in the day.