North Korea's Jan. 6 nuclear test did not expand its technical capability, but the US government is keeping a close eye on Pyongyang's efforts to develop a thermonuclear warhead capable of reaching the United States, the head of the U.S. Missile Defense Agency said on Tuesday.

"I would assess that their technical capability has not increased," Vice Admiral James Syring told an event hosted by the Center for Strategic and International Studies. "That said, everything that they're doing continues to be alarming and provoking. ... We continue to watch it closely."

Syring gave no further details on what was North Korea's fourth nuclear test.

The United States has made no major changes in efforts to identify, track and intercept potential North Korean missile threats as a result of the latest test, he said. "If it was warranted, you would see our program change," he said. "We are absolutely on the right path to stay ahead of that threat."

He said the Missile Defense Agency would have 37 ground-based interceptors in place in Alaska and California by the end of the year, and 44 such interceptors by the end of 2017. Then-Defense Secretary Chuck Hagel ordered 14 additional interceptors to be put in place in March 2013 after North Korea's third nuclear test.

Nuclear experts say North Korea likely gained data and practical know-how from the test. They reject North Korea's assertion that it detonated a hydrogen bomb.

READ: North Korea says conducted 'successful' hydrogen bomb test

In an H-bomb, conventional explosives compress and detonate a conventional fission bomb, triggering a powerful secondary fusion device. The process likely used by North Korea, called "boosting," involves an intermediate device that uses a hydrogen isotope to vastly increase the explosive power of an old-fashioned fission bomb, the experts told Reuters.

Boosting is key to miniaturising a thermonuclear weapon, and Pyongyang must master miniaturization in order to build a warhead small enough to fit atop a ballistic missile that can reach the United States or other distant targets, experts said.


'SMALLER AND LIGHTER'

Siegfried Hecker, a former director of the Los Alamos National Laboratory, said the test "will certainly allow North Korea to increase the sophistication of its nuclear arsenal - specifically, to make the nuclear bombs smaller and lighter."

Admiral Bill Gortney, commander of U.S. Northern Command, has said he believes North Korea already has the ability to miniaturise nuclear weapons and place them on missiles that could reach the United States.

North Korea is likely moving along the miniaturization path, developing a boosting process and reducing the amount of chemical explosive needed to compress the core, experts say.

"On those two levels, they can achieve some real weight savings," said Hans Kristensen, director of the Federation of American Scientists' Nuclear Information Project.


North Korea has shown off two versions of a ballistic missile that appear to be of a type that could reach the U.S. West Coast, but there is no evidence the missiles have been tested.

North Korea has also tested a space-launch vehicle that could be modified to work as an intercontinental ballistic missile. It also has released a video of a what it said was a successful test of a submarine-launched missile.

Pyongyang has maintained its nuclear programs despite broad international sanctions, helped by lax enforcement of restrictions by its neighbour and main ally China.