"There is a preliminary analysis that suggests that the event in North Korea was consistent with a nuclear explosion," a US official said on Friday.

The official added that no final determiniation had been made about the nature on Monday's blast but the findings are the strongest evidence yet that a nuclear device was detonated.

Official findings are expected in several days.

One congressional official said that radioactive material was found in an air sample collected on Wednesday. It was one of several tests that have been carried out to determine the validity of North Korea's claims.

"The betting is that this was an attempt at a nuclear test that failed," another official said. "We don't think they were trying to fake a nuclear test, but it may have been a nuclear fizzle."

Sensitive information

The officials spoke on condition of anonymity because of the sensitive nature of the information.

The administration briefed key members of Congress about the preliminary test results. One official said the sample was collected above Qunggye, near the area of the claimed nuclear test.

North Korea's neighbours, Japan and South Korea, have both said they have not detected any unusual radioactivity since the test was announced.

"Our latest observations up to Saturday, which are aimed at  monitoring radiation levels harmful to human beings, found nothing abnormal," Hiroshi Tsuboi, an official from Japan's taskforce dealing with possible fallout, said.
  
Radiation monitoring

"We've heard news reports the US government detected radioactive elements, but what we can do is to conduct our own monitoring."

South Korea said it was still assuming Monday's seismic event was caused by a nuclear test despite not registering any fallout.

"We examined air and water samples, but we have not detected any radioactive contamination in South Korea following the nuclear test," Yun Ju-Yung, a senior researcher at the Korea Institute of Nuclear Safety, said.

Japan and South Korea are monitoring radiation levels in their countries, while the US samples were taken by a military drone over North Korea the day after the test.