North Korea blast twice 2009 size - monitoring agency

Executive secretary Tibor Toth (L) and director of the international data centre division Lassina Zerbo of the Comprehensive Nuclear-Test Ban Treaty Organization (CTBTO) address a news conference in Vienna February 12, 2013.

VIENNA (Reuters) - The seismic magnitude of an "explosion-like" event in North Korea on Tuesday was roughly twice as large as that of a 2009 nuclear test in the country, an international nuclear test monitoring agency said.

"We can assume this is roughly twice as big in magnitude," Lassina Zerbo, director of the international data centre division of the Comprehensive Nuclear-Test Ban Treaty Organization (CTBTO) told a news conference.

North Korea said earlier it had carried out a third nuclear test on Tuesday.

The CTBTO's assessment was based on its measurement of a seismic event of 5 on the Richter scale - a figure it raised from an initial 4.9 - versus 4.5 in 2009, and 4.1 in 2006.

The U.S. Geological Survey said earlier that a seismic event measuring 5.1 had occurred.

Zerbo said the "smoking gun" of any nuclear test would be the potential detection of radionuclide traces.

"We cannot say anything about that before two to three days minimum," he said.

The CTBTO said indications were that the location of the event was "largely identical" with the two previous nuclear tests. "As with the two previous nuclear tests, the signal was emitted from close to the surface," it said in a statement.

(Reporting by Michael Shields and Fredrik Dahl; Writing by Georgina Prodhan; Editing by Jon Hemming)