Additional nuclear and missile tests would be required for North Korea to master the technology to put a nuclear warhead on a long-range ballistic missile, a recent U.S. congressional research report said.

The Congressional Research Service made the assessment in the Jan. 15 report, titled "North Korea: U.S. Relations, Nuclear Diplomacy, and Internal Situation," saying the official position of the director of national intelligence is that the North "has not yet demonstrated the full range of capabilities necessary for a nuclear armed missile."

"Miniaturization likely would require additional nuclear and missile tests," the report said. "Perhaps the most acute near-term threat to other nations is from the medium-range Nodong missile, which could reach all of the Korean Peninsula and some of mainland Japan."

"The report also noted the White House cast doubts over the North's claims of a successful hydrogen bomb test.

"The U.S. government confirmed that the underground explosion was a nuclear test, but a White House spokesman said that initial data was 'not consistent' with North Korean claims of detonating a full-fledged thermonuclear hydrogen bomb," the report said, adding that the North's three previous tests involved fission devices.

It also raised the possibility that the North could have tested a "boosted fission weapon," saying testing such a device could be the next step after testing fission weapons on the path to developing a hydrogen bomb. Boosted fission weapons would also be lighter and smaller than a fission weapon with comparable yield, it said.

The report also said the North's development of submarine-launched ballistic missiles is not considered an "imminent" threat, saying the December test of an SLBM was a "failure."

"SLBM technology is extremely difficult to develop, and the reports of testing do not indicate that North Korea's prototype ballistic missile submarines represent an imminent threat," it said.

It cited an unidentified expert on North Korean military affairs as saying, "under optimal conditions this (SLBM capability is) an emerging regional threat rather than an imminent threat. It does not represent an emerging intercontinental threat." (Yonhap)