The classified report was compiled last month but its contents were made public on Thursday during a congressional hearing by Representative Doug Lamborn, a Republican from Colorado.
Mr Lamborn read from a declassified summary and asked General Martin Dempsey, the US's most senior military officer, to comment on the finding.
Gen Dempsey declined to address the issue but within hours the Pentagon released a statement denying that North Korea had achieved a functioning nuclear weapon.
"While I cannot speak to all the details of a report that is classified in its entirety, 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 passage," said George Little, the Pentagon spokesman.
The US denial was followed quickly by the South Korean defence ministry, which said the North had still not developed a warhead small enough to place on top of a a missile.
"Our military's assessment is that the North has not yet miniaturised," ministry spokesman Kim Min-seok told a news briefing.
"North Korea has conducted three nuclear tests but there is doubt whether it is at the stage where they can reduce the weight and miniaturise to mount on a missile."
The DIA is one of a number of US intelligence agencies monitoring North Korea's nuclear progress and it is believed that most of its counterparts remain unconvinced Pyongyang has developed a working weapon.
In the build-up to the Iraq War, DIA inaccurately concluded that Saddam Hussein's Iraq possessed weapons on mass destruction.
The Pentagon's denials came hours after President Barack Obama tried to calm weeks of growing tension with the Communist dictatorship.
"Now is the time for North Korea to end the kind of belligerent approach that they've been taking, and to try to lower temperatures," Mr Obama said following a meeting with Ban Ki-Moon, the UN Secretary General. "Nobody wants to see a conflict on the Korean Peninsula."
Mr Ban, the former foreign minister of South Korea, also made a direct appeal in Korean to Kim Jung-un, the North Korean leader, to end his provocative actions.
"In order to restore peace and unification to our Korean Peninsula, so we can resolve all pending issues through dialogue, as Secretary General of the UN and as a citizen of Korea, I sincerely ask that you end the recent provocative actions and return to dialogue," he said on CNN.
John Kerry, the US secretary of state, will arrive in Seoul today tasked with both reassuring South Korea of America's commitment and ensuring the democracy does nothing to provoke further action by the North.
US intelligence officials offered a brief character portrait of Mr Kim, about whom little, even his age, his is known.
"He impresses me as impetuous – not as inhibited as his father became about taking aggressive action," said James Clapper, the Director of National Intelligence.
Mr Clapper said the North Korean leader was "underestimating the Chinese frustration" with his provocative rhetoric but added that he clearly been influenced by his time at school in Switzerland and recognised that his country's economy was in "an extremis situation".
