The United States on Thursday insisted North Korea did not have nuclear-armed missiles as officials sought to play down a leaked intelligence report warning Pyongyang had likely succeeded in building atomic warheads that could be fit on a ballistic missile.

Pentagon spokesman George Little said "it would be inaccurate to suggest that the North Korean regime has fully tested, developed, or demonstrated the kinds of nuclear capabilities referenced in" the intelligence report.

The statement came after a lawmaker read out the findings of a Defense Intelligence Agency (DIA) report at a congressional hearing earlier Thursday that appeared to signal a shift in Washington's view of North Korea's nuclear program.

The DIA assessment said US analysts had "moderate confidence the North currently has nuclear weapons capable of delivery by ballistic missiles" but that the reliability of the weapons was "low."

The intelligence report marked the first time the US government had suggested North Korea may have succeeded in a years-long quest to miniaturize a nuclear device so that it could be placed on a missile.

Defense Secretary Chuck Hagel and the US military's top officer, General Martin Dempsey, appeared caught off-guard when the report's findings were disclosed at a hearing of the House Armed Services Committee.

After the hearing, administration officials scrambled to explain what appeared to be a change in the government's portrayal of the North Korean threat.

A senior US official, who spoke on condition of anonymity, told AFP that the administration had not changed its evaluation that Pyongyang was still not at a point where it could deliver a nuclear warhead on a ballistic missile.

"The North Koreans have never demonstrated this capability and we don't believe they are able to now," the official said.

The confusion over the spy agency report came against the backdrop of mounting tensions on the Korean peninsula, with Pyongyang issuing bellicose threats and preparing to possibly launch medium-range missiles.

The Pentagon, which has bolstered missile defenses around the Korean peninsula, said it was closely watching North Korea amid speculation the regime would fire conventional missiles in the run-up to national celebrations on April 15th.

"The United States continues to closely monitor the North Korean nuclear program and calls upon North Korea to honor its international obligations," Little said. (AFP)