VIENNA (REUTERS) - A nuclear test monitoring agency said on Tuesday an unusual seismic event detected in North Korea showed "clear explosion-like characteristics" and suggested it occurred at about the same place as previous tests by the North.

Mr Tibor Toth, executive secretary of the Comprehensive Nuclear Test Ban Treaty Organisation (CTBTO), said the seismic event's location was "roughly congruent with" nuclear tests carried out by North Korea in 2006 and 2009.

"For now, further data and analysis are necessary to establish what kind of event this is," he said in a statement.

"If confirmed as a nuclear test, this act would constitute a clear threat to international peace and security, and challenge efforts made to strengthen global nuclear disarmament and non-proliferation, in particular by ending nuclear testing," he added.

A Western diplomat in Vienna, where the CTBTO is based, said that "it certainly looks like a nuclear test. We are trying to confirm that." The test-ban treaty has not yet taken effect because not all holders of nuclear technology have ratified it.

But the organisation already monitors possible breaches, deploying more than 270 stations worldwide to look out for signs of atomic tests, including seismic waves and radioactive traces.