Pyongyang insisted that South Korea's nuclear activities should be discussed as the most important issue at six-party talks on ending North Korea's nuclear weapons drive.

The International Atomic Energy Agency last week reproached South Korea for its undeclared experiments in which small amounts of weapons-grade nuclear material were produced.

But the watchdog allowed South Korea to escape referral to the UN Security Council for possible sanctions.

"Under this situation the DPRK (North Korea) is left with no option but to increase its nuclear deterrent force," a North Korean foreign ministry spokesman said in a statement published by the official KCNA news agency.

"If the IAEA does not settle the secret nuclear experiments of South Korea in an understandable manner, this issue will stand out as the most important issue at the six-party talks pending a top priority discussion," he said.

The spokesman accused the United States of downplaying South Korea's nuclear experiments while demanding North Korea admit what it called its "non-existent" uranium enrichment program.

North Korea has admitted to firing up its mothballed plutonium-based nuclear reactor but has denied running a covert uranium-enrichment program.

"Double standards as regards the nuclear issues of the North and the South of Korea can never be allowed under any circumstances and it does not stand to reason that the DPRK alone should work for denuclearization," the spokesman said.

South Korea admitted in September that its scientists produced small amounts of plutonium in 1982 and enriched uranium in 2000 without informing the nuclear watchdog.

The revelations embarrassed both Washington and Seoul which are trying to pressure North Korea to end its nuclear program.

Three rounds of six-party talks -- between the United States, China, the two Koreas, Japan and Russia -- have taken place since the stand-off over North Korea's nuclear weapons drive began in October 2002.

The North boycotted a fourth round scheduled for September, citing Washington's "hostile" policy towards Pyongyang and Seoul's nuclear experiments.

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