N.Korea test "slightly bigger" than in 2006

VIENNA (Reuters) - A nuclear test claimed by North Korea on Monday is "slightly bigger" than the one carried out by Pyongyang in 2006, the Vienna-based nuclear test ban treaty organization said, contradicting reports of a bigger blast.

Russia's Itar-Tass agency quoted a source in Russia's defense ministry as saying the test had a force of about 20 kilotonnes, compared with a generally accepted strength of about one kilotonne in 2006.

A kilotonne is equivalent to 1,000 tonnes of TNT.

"The event's magnitude is slightly higher than in 2006, measuring 4.52 on the Richter scale, while in 2006 it was 4.1," the Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO) said in a statement.

The tremor took place just below the ground, "within a couple of kilometers" from the previous test and has an estimated "low single digit" kilotonne range, the CTBTO said.

"This is the wrong step in the wrong direction," CTBTO Executive Secretary Tibor Toth told reporters.

The CTBTO, the world's independent body for monitoring possible breaches of the test ban, has collected data from 39 seismic stations around the world and is awaiting detail on possible radioactive particles and noble gases.

New stations close to the DPRK, in China, Japan and Russia had helped speed up readings and make them more precise, the CTBTO said.

In a statement earlier on Monday Toth said it was "a serious violation of the norm established by the Comprehensive Nuclear-Test-Ban Treaty (CTBT) and...deserves universal condemnation."

The CTBT is a global treaty which prohibits all nuclear explosions. The pact cannot enter into force before it is ratified by all 44 states listed that took part in the 1996 negotiations and have nuclear power or research reactors.

Of the five nuclear weapon states, the United States and China still have to ratify it.

The CTBTO said its initial North Korea findings were precise enough to request an on-site inspection under the treaty's rules. This is a step which could become automatic when the pact comes into force.