ST GALLEN, Switzerland (Reuters) - The United Nations nuclear chief said on Friday he would not be surprised if North Korea were to carry out a new nuclear test, amid speculation the secretive Asian state is preparing to conduct the third such explosion since 2006.

"We don't have inspectors on the ground. We are following the situation carefully. We do not have particular knowledge or information but if a nuclear test takes place I would not be surprised," Yukiya Amano, director general of the International Atomic Energy Agency (IAEA), said at a conference in the Swiss town of St Gallen.

The five permanent members of the U.N. Security Council urged North Korea on Thursday to refrain from any new nuclear tests.

North Korea, which tested plutonium devices in 2006 and 2009, has almost completed preparations for a third nuclear test, a senior source with close ties to Pyongyang and Beijing told Reuters last month.

The isolated state sacrificed the chance of closer ties with the United States after it tried to test-launch a long-range rocket on April 13 and was censured by the U.N. Security Council.

Pyongyang has long argued that in the face of a hostile United States, which has military bases in South Korea and Japan, it needs a nuclear arsenal to defend itself.

(Reporting by Emma Farge; writing by Fredrik Dahl; Editing by Janet Lawrence and Alessandra Rizzo)