The White House said Friday that North Korea has not demonstrated the ability to deploy nuclear missiles, downplaying an intelligence report from the day before.

"North Korea has not demonstrated the capability to deploy a nuclear armed missile," spokesman Jay Carney said, while adding that "there's no question that this is a situation that requires the US to take necessary prudent measures and that is what we've done."

A US lawmaker on Thursday quoted a report from the military's Defense Intelligence Agency saying Pyongyang may have succeeded in the technologically difficult task of making a nuclear weapon small enough to fit on a warhead.

"DIA assesses with moderate confidence the North currently has nuclear weapons capable of delivery by ballistic missiles," Representative Doug Lamborn said, reading from what he said was an unclassified portion of the assessment.

"However, the reliability will be low," he added.

The Pentagon threw cold water on the assessment, with spokesman George Little saying "it would be inaccurate to suggest that the North Korean regime has fully tested, developed, or demonstrated the kinds of nuclear capabilities referenced" by the lawmaker.

Amid mounting tensions on the Korean peninsula, intelligence officials in Seoul say the North has two mid-range missiles ready for imminent launch from its east coast.

The United States, along with its allies South Korea and Japan, has bolstered missile defenses in the region with naval ships equipped with anti-missile weaponry, a floating radar and interceptors on Guam.

American commanders say their forces would be ready to shoot down a North Korean missile if it threatened South Korea, Japan or US bases in Guam.

Secretary of State John Kerry, currently in Seoul, demanded North Korea scrap the expected missile test and tone down its rhetoric, while the top US diplomat also backed new peace overtures by Seoul and appealed for calm.