North Korean negotiators said the dispute over their country's nuclear activity can only be solved through dialogue with the United States, a South Korean envoy said Wednesday after returning from Pyongyang.

The envoy, Lim Dong-won, had hoped to meet with North Korean leader Kim Jong Il to try to personally dissuade him from pursuing nuclear weapons development. But the meeting did not take place and North Korean officials said Kim was not in the capital at the time of Lim's visit.

"North Korean officials repeated that the nuclear issue is a matter that concerns North Korea and the United States," said Lim, who delivered a letter from South Korean President Kim Dae-jung to the North Korean leader through aides.

Communicating through aides, Kim Jong Il thanked his South Korean counterpart for the letter and promised to study its "warm advice," Lim told a nationally televised news conference.

Lim ruled out any quick solution to the North's nuclear dispute, saying that it will be "a very long and gradual process."

"The fundamental solution of the nuclear issue can be achieved only when the country suspected of building nuclear weapons doesn't feel any security threats and builds relationships of trust with other countries," he said.

"There can't be 100 percent verification of a nuclear weapons program," Lim said, speaking in general of the difficulty of confirming that a nation is not secretly developing atomic arms.

While calling for talks with Washington, North Korea said Wednesday that the United States was trying to stifle it through economic and political pressure in the same way that a snake smothers and consumes its victim.

In a commentary on its news agency, KCNA, North Korea charged that Washington was using the dispute over North Korea's nuclear activities as a pretext to destroy the communist country.

"This strategy is also dubbed a 'serpent' strategy as it is to be carried out in the way a serpent does i.e. swallowing up the object after strangling it," KCNA said.

In Washington, President Bush said in a State of the Union speech that the United States and other countries would not be "blackmailed" into granting concessions to North Korea because of its nuclear weapons development.

The United States is working with South Korea, Japan, China and Russia "to find a peaceful solution," Bush said in the speech that was heard Wednesday morning in Asia, "and to show the North Korea government that nuclear weapons will bring only isolation, economic stagnation and continued hardship."

Bush described Kim Jong Il's government as an "oppressive regime" that "rules a people living in fear and starvation." Last year Bush labeled North Korea part of an "axis of evil" along with Iraq and Iran.

North Korea is demanding a nonaggression treaty with the United States before it gives up its nuclear ambitions. Washington has ruled out a formal treaty but said it can provide a written security guarantee. Washington wants to bring the North's nuclear issue before the U.N. Security Council, which could eventually impose sanctions on Pyongyang.

In meetings with Lim, Kim Yong Sun, a key aide to the North Korean leader, reiterated Pyongyang's stance that the nuclear issue is entirely with the United States, the North's KCNA news agency reported.

"It is the only way of most fairly solving the `nuclear issue' on the Korean Peninsula for the DPRK and the U.S. to have direct talks on an equal footing," the report said. DPRK stands for the Democratic People's Republic of Korea.

South Korean officials said its delegation achieved "some degree of success" after conveying its anti-nuclear position clearly to North Korea while hearing the North's response. The delegation also met Kim Yong Nam, the North's ceremonial head of state and No. 2.

The dispute was sparked in October when U.S. officials said North Korea had admitted having a nuclear program based on uranium enrichment in violation of a 1994 agreement with the United States. Washington and its allies suspended oil shipments to North Korea, which then expelled U.N. nuclear inspectors and pulled out of a global nuclear arms control treaty.

The envoy's visit — coming after a round of North-South Cabinet-level talks last week in Seoul — was part of South Korea's efforts to seek a negotiated end to the North's suspected nuclear weapons development.