An international nuclear test monitoring agency said on Tuesday that it had detected an "unusual seismic event" in the Democratic People's Republic of Korea (DPRK), following weeks of speculation that a nuclear test was imminent in the country.

"We have detected an unusual seismic event in North Korea," said Annika Thunborg, a spokeswoman for the Comprehensive Nuclear-Test Ban Treaty Organization, an agency set up to monitor compliance with a ban on nuclear tests that has yet to come into effect.

She gave no further details.