There is increased activity at North Korea's known nuclear test site, a South Korean news report said on Thursday, suggesting Pyongyang is gearing up for a new test as it has threatened in response to tightened U.N. sanctions. Impoverished North Korea, whose only nuclear test in October 2006 led to U.N. financial and trade sanctions, could be ready to test another nuclear device in a matter of weeks, experts have said.

"Underground nuclear tests are hard to predict and you can't tell when exactly a nuclear test would be possible, but we think the North is ready to conduct a test in a near future if it wants to," the Chosun Ilbo daily quoted a government source as saying. South Korean authorities are monitoring increased and steady activity at the Phunggye-ri site in the North Hamgyong province where the North conducted the 2006 test, the newspaper said.

The North also appears to have stepped up construction at a new long-range missile launch site in the west that had been expected originally to be completed by the end of the year, the government source was quoted as saying.

South Korea's Foreign Ministry and the spy agency declined to comment on the report. Last week, the North threatened a new nuclear test unless the U.N. Security Council apologised and withdrew the sanctions, tightened after it launched a long-range rocket in April.

Analysts say North Korea wants to play out its test preparations, many of which can be seen by U.S. spy satellites, for as long as possible to increase leverage in negotiations aimed at ending its efforts to build a nuclear arsenal. Talks among six countries, which also include South Korea, Japan, China and Russia, have been deadlocked over disagreement on how to inspect the North's nuclear arms programme and how to compensate Pyongyang for dismantling it.

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