WASHINGTON/SEOUL, April 12 – A Pentagon spy agency has concluded with "moderate confidence" that North Korea has the ability to launch nuclear-armed ballistic missiles but the weapons would probably be unreliable, a U.S. lawmaker said on Thursday.

A Pentagon spokesman later said it was "inaccurate" to suggest North Korea had a demonstrated ability to launch a nuclear missile. That view is supported by civilian experts.

The assessment by the Defense Intelligence Agency (DIA) was made public by Republican Representative Doug Lamborn as he questioned Pentagon officials about North Korea's nuclear weapons programme during a hearing of the House of Representatives Armed Services Committee in Washington.

"DIA assesses with moderate confidence the North currently has nuclear weapons capable of delivery by ballistic missiles, however the reliability will be low," Lamborn said, quoting from a DIA report entitled "Dynamic Threat Assessment 8099: North Korea Nuclear Weapons Program (March 2013)".

Effectively confirming the assessment, 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.

North Korea has threatened Washington and Seoul with nuclear war, but most experts say Pyongyang has no intention of starting a conflict that would likely bring its own destruction.

South Korea, Japan and the United States are on alert for the test launch of a medium-range missile ahead of the anniversary on Monday of the birth of North Korean founder Kim Il-Sung, the grandfather of current leader, Kim Jong-un.

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.

Meanwhile, U.S. Secretary of State John Kerry arrives in Seoul later on Friday on a trip that will include Japan and China. North Korea is expected to dominate his talks.

"Inaccurate"

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 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 miniaturizing a nuclear weapon to be placed on a long-range missile, a capability the United States, Russia, China and others achieved decades ago.

North Korea last tested a long-range rocket in December. It launched the rocket into space for the first time but experts say it has not mastered the technology to ensure the re-entry of any ballistic missile into the earth's atmosphere.

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 said the agency reached the conclusion in a mostly classified March 2013 report. He did not say what range the nuclear-capable North Korean missiles might have.

Tensions on the Korean peninsula spiked after North Korea's third nuclear test on Feb. 12. The United Nations imposed fresh sanctions in response, which infuriated Pyongyang.

The North is also angry about weeks of joint South Korean-U.S. military exercises. About 28,000 U.S. forces are based permanently in South Korea.

U.S. spy agencies believe the rhetoric from North Korea is mainly an effort by new leader Kim to demonstrate he is in command, U.S. National Intelligence Director James Clapper said on Thursday.

Despite recent threats, North Korea has been welcoming foreign visitors for Monday's celebrations.

South Korean president offers talks

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.

The details of North Korea's weapons programmes known to U.S. and other intelligence agencies remain classified. There also appear to be gaps in that knowledge due to North Korea's highly secretive nature.

Some U.S. official and private weapons experts say North Korea may have succeeded in designing, and possibly building, a miniaturized nuclear device that could be fitted aboard medium-range missiles known as the Nodong.

This is in dispute, however. Even if Pyongyang has developed such a warhead, there are serious doubts about whether North Korea would be able to test it enough to ensure it worked.

Medium-range missiles such as the Nodong might be able to reach U.S. allies South Korea and Japan, as well as Okinawa, where there is still a large U.S. military presence.

In response to the recent threats from North Korea, Washington has been beefing up missile defences in Alaska and Guam in the Pacific.