Preliminary U.S. intelligence estimates have concluded that North Korea's latest nuclear test involved a small explosion that could be a component of a larger-scale thermonuclear device, a U.S. news report said Thursday.

Washington Free Beacon report came amid growing skepticism about Pyongyang's claims that it successfully carried out its first hydrogen bomb test, with the White House saying "initial analysis is not consistent" with the North's claims.

The report said that U.S. officials familiar with intelligence reports of Wednesday's test estimated the low yield of the detected blast to be between 5 kilotons to 7 kilotons -- far less than would be detected in a two-stage thermonuclear blast, or hydrogen bomb.

The report, however, did not provide any further specifics about the "component" of a thermonuclear device.

"Initial reports indicate the North Koreans may be bragging a little bit too much," one unidentified official was quoted in the report as saying of the claims of a hydrogen bomb test.

U.S. intelligence authorities also believe that the North's test had two main goals, the report said.

One of them was to back up leader Kim Jong-un's statement last month that the country developed an H-bomb and the other purpose was to persuade China, North Korea's main patron, to back off pressuring the regime to abandon its nuclear program, the report said.

U.S. nuclear experts have expressed doubts about North Korea's claims, saying its yield is too low to believe that such an advanced bomb, way more powerful than conventional nuclear weapons, has been detonated.

"It is likely that this was not a test of what in the popular literature is interpreted as an H-Bomb, namely a two-stage fission-fusion weapon developed by the major nuclear-weapon states capable of obtaining explosive yields of hundreds or thousands of kilotons," said David Albright, a nuclear expert heading the Institute for Science and International Security in Washington.

South Korea's spy agency believes that the yield of the test was 6 kilotons and a 4.8-magnitude temblor was detected. In comparison, the yield of the nuclear device North Korea tested in

2013 was 7.9 kilotons and it caused a 4.9-magnitude earthquake.

"If North Korea had indeed tested this type of H-bomb, the device's yield would be expected to be many tens of kilotons, at least," Albright said.

Jeffrey Lewis, a nonproliferation expert and director of the East Asia Nonproliferation Program at the James Martin Center for Nonproliferation Studies, said that the North is believed to have tested a "boosted fission device," which is smaller in size but greater in yield than conventional nuclear weapons.

"I think this is the most likely explanation given the yield of the event and the status of North Korea's nuclear program," he said in a report carried by 38 North. "Boosting involves using deuterium-tritium gas to increase the efficiency of a nuclear explosion."

China's first test of a boosted fission device came in at 15 kilotons, he said.

"Boosting would allow North Korea to make more efficient use of its existing stockpile of fissile material without sacrificing yield," he said. (Yonhap)