A Pentagon spy agency has concluded with "moderate confidence" that North Korea has developed a nuclear-tipped ballistic missile, an assessment swiftly dismissed by several U.S. officials and South Korea.

The Defense Intelligence Agency (DIA) said such a weapon would probably be unreliable. Its assessment, made public by a U.S. lawmaker in Washington, comes amid threats of war by North Korea and just hours before U.S. Secretary of State John Kerry arrives in Seoul on a visit to the region that will include stops in China and Japan.

South Korean and U.S. officials say Pyongyang appears set to test launch a medium-range missile as a show of strength ahead of the anniversary on Monday of the birth of North Korean founder Kim Il-Sung.

South Korea's Defence Ministry however said it did not believe North Korea could mount a nuclear warhead on a missile. Despite the DIA report, the Pentagon's spokesman and the U.S. national intelligence director both said it was "inaccurate" to infer Pyongyang had the proven ability to launch a nuclear missile.

The DIA was criticised after the start of the Iraq war in 2003 for being too bullish in predicting Baghdad might have weapons of mass destruction. Its conclusion about North Korea follows more than a month of rising tension on the Korean peninsula.

North Korea, claiming the United States is planning to invade, has threatened Washington and Seoul with nuclear war, although most experts say Pyongyang has no intention of starting a conflict that would likely bring its own destruction.

"DIA assesses with moderate confidence the North currently has nuclear weapons capable of delivery by ballistic missiles, however the reliability will be low," Republican Representative Doug Lamborn said during a hearing of the House of Representatives Armed Services Committee in Washington.

He was quoting a report entitled "Dynamic Threat Assessment 8099: North Korea Nuclear Weapons Program (March 2013)".

A U.S. official said the quotation cited by Lamborn was in a section that had been erroneously marked unclassified. The study, dated last month, appeared to be the first time the agency had reached such a conclusion.

Question of Miniaturisation

Seoul played down the report.

"Our military's assessment is that the North has not yet miniaturised," South Korean Defence 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."

Pyongyang has frequently cited the U.S.-led invasion of Iraq as a reason it needed nuclear weapons, saying that without them, Washington would seek to topple its government too.

The United Nations sanctioned North Korea for a nuclear test on Feb. 12, its third, sparking a furious response from Pyongyang. The North has also called annual military drills between U.S. and South Korean forces a "hostile" act.

North Korea has stationed as many as five medium-range missiles on its east coast, according to defence assessments by Washington and Seoul. South Korean and U.S. officials believe it is preparing to launch a Musudan missile, whose range of 3,500 km (2,100 miles) or more would put Japan within striking distance and may threaten Guam, home to U.S. military bases.

'Inaccurate'

The debate about North Korea's nuclear ability focuses on whether it has a warhead small enough to mount on a missile and whether it can then ensure that missile re-enters the earth's atmosphere.

North Korea last tested a long-range rocket in December. It launched the rocket into space for the first time but the rocket did not successfully re-enter.

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 passage" of the DIA report.

The conclusion of the DIA was not shared by the wider U.S. intelligence community, U.S. National Intelligence Director James Clapper said in a statement. The strong consensus inside the U.S. government is that North Korea does not yet have a nuclear device that would fit longer-range missiles that conceivably could hit the U.S. mainland.

Civilian experts have also said there was no evidence North Korea had tested the complex art of miniaturising a nuclear weapon to be placed on a long-range missile, a capability the United States, Russia, China and others achieved decades ago.

Greg Thielmann, a former State Department intelligence analyst now with the Arms Control Association advocacy group, said that while he did not have access to the classified material cited in Congress, what was said publicly about DIA's assessment sounded quite tentative.

"It really says to me that this is a speculative statement," Thielmann said. "Moderate (confidence) is higher than low confidence but it doesn't say they know very much."

Lamborn, the congressman, said the DIA reached the conclusion in a mostly classified March 2013 report. He did not say what range the nuclear-capable North Korean missiles might have. U.S. spy agencies believe the threats of war from North Korea mainly represent an effort by new leader Kim Jong-un to demonstrate he is in command, Clapper said on Thursday.

New South Korean President Park Geun-hye said late on Thursday she was open to resume dialogue with the North and would continue to offer humanitarian aid. Her long-standing policy is that the North needs to abandon its nuclear programme before it gets aid.