* Magnitude of blast measured at 4.9 - CTBTO agency
* Would mean explosion larger than 2006, 2009 nuclear tests
* Location very close to previous tests
* Occurred shortly before 0300 GMT
By Fredrik Dahl
VIENNA (Reuters) - An explosion that North Korea said was a nuclear test had a magnitude of 4.9, bigger than similar tests the Asian state carried out in 2006 and 2009, a monitoring agency said on Tuesday.
The Comprehensive Nuclear Test Ban Treaty Organisation (CTBTO) estimated the blast occurred at 0258 GMT, at or close to where the North conducted its previous two nuclear tests.
The first, in 2006, measured 4.1 on the same scale. A second three years later registered 4.52.
The CTBTO, which has more than 270 facilities worldwide to detect possible nuclear tests, initially said it had detected an "unusual seismic event", which it later called an "explosion."
"The explosion measured 4.9 on the Richter scale, which was higher than the North Korean nuclear tests in 2006 and 2009," it said.
North Korea said the test, which drew swift and widespread condemnation, had "greater explosive force" than the two previous tests, and that it had used a miniaturized device.
If it was such a device, "that suggests a weapon that could more easily be delivered by missiles, something that is especially concerning to the U.S., Japan, South Korea and others," said Shashank Joshi, a senior fellow at the Royal United Services Institute.
Tibor Toth, CTBTO executive secretary, said in a statement that North Korea's action "constitutes a clear threat to international peace and security and challenges efforts to strengthen global nuclear disarmament and non-proliferation."
He added: "I'm gravely concerned by this action, which deserves universal condemnation ... Nuclear testing needs to end once and for all."
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.
But the preparatory organisation already 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 signs that would confirm beyond doubt that a nuclear 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.
The U.N. nuclear watchdog, like the CTBTO based in Vienna, expressed "deep regret" about the North's announcement it had carried out another nuclear test.
"This is ...in clear violation of U.N. Security Council resolutions," Yukiya Amano, Director General of the International Atomic Energy Agency (IAEA), said in a statement.
(Editing by John Stonestreet)