* Magnitude of blast measured at 5 - test monitoring agency
* Reading suggests it was larger than 2006 and 2009 tests
* Sends message but actual threat to U.S. still seen distant (Adds details on magnitude, U.S. nuclear expert)
By Fredrik Dahl
VIENNA, Feb 12 (Reuters) - North Korea's third nuclear test may be aimed at showing its foes it can develop a warhead for a missile but it is still a long way from being able to threaten the United States, proliferation experts say.
The secretive Asian state is widely believed to be trying to develop a device that is compact and light enough to fit on top of a missile. North Korea said the test, which was condemned by world powers, had used a miniaturised device.
Comprehensive Nuclear Test Ban Treaty Organisation (CTBTO) executive secretary Tibor Toth said the action "constitutes a clear threat to international peace and security and challenges efforts to strengthen global nuclear disarmament and non-proliferation".
The international monitoring agency said the "explosion-like" event that North Korea described as a nuclear test had a seismic magnitude of 5, around twice as large as a test it carried out in 2009 and much bigger than one in 2006.
"It seems as if Pyongyang wants to send the message - true or not - that it can employ a missile with a nuclear warhead and that previous problems with their nuclear tests have been overcome," Jim Walsh of Massachusetts Institute of Technology (MIT) said.
Building a compact, light device has long been considered out of the North's technical reach, the Stockholm International Peace Research Institute (SIPRI) said.
The results of the test are not known, but SIPRI said a successful operation would bring North Korea one step closer to having the capability of building a long-range ballistic missile which could deliver a nuclear weapon.
The more immediate concern is the chance that the North could mount and deploy a nuclear warhead on its shorter range Nodong missile, said U.S. expert David Albright.
"North Korea probably cannot deploy a warhead on an ICBM (intercontinental ballistic missile). However, with additional effort and time, North Korea will likely succeed in developing such a warhead too," Albright said.
North Korea is banned under U.N. Security Council resolutions from developing nuclear and missile technology.
U.S. intelligence agencies were analysing the event and had found that North Korea probably conducted an underground nuclear explosion with a yield of "approximately several kilotons", the Office of the Director of National Intelligence said.
NO LIES ON TEST
The North's ultimate aim, Washington believes, is to design a ballistic missile capable of carrying a nuclear warhead that could hit the United States. Pyongyang says the programme is aimed merely at putting satellites in space.
However, North Korea still had a long way to go before it could credibly threaten the United States with nuclear weapons, said Daryl Kimball from the Arms Control Association, a U.S.-based research and advocacy group.
"It is likely to be years away from fielding an ICBM which could deliver a nuclear warhead to the U.S. mainland," he said.
An international test-ban treaty was negotiated in the 1990s but has not yet taken effect because not all holders of nuclear technology have ratified it. The Vienna-based CTBTO monitors possible breaches, looking out for signs of atomic tests, including seismic waves and radioactive traces.
Experts say it can take days or more to detect possible radioactive "smoking gun" evidence that a test had taken place.
Seen as a cornerstone of efforts to free the world of atomic bombs, the test ban treaty enjoys wide support around the world. But of the five officially recognised nuclear weapon states, the United States and China have yet to ratify it.
"Though confirmation will take some time, given the seismic signature and the important fact (North Korea) has never lied when it comes to nuclear tests, I think we can take them at their word and assume this was the explosion of a nuclear device," a Western diplomat in the Austrian capital said.
Kimball said the test was an embarrassment for China's leadership and Pyongyang may have jeopardised the aid and diplomatic support it receives from Beijing.
China criticised the previous tests but did not roll back on aid. But Beijing had signalled that if North Korea undertook further tests, it would not hesitate to reduce assistance. (Editing by Alison Williams)