Seoul, South Korea ? North Korea accused the United States on Wednesday of adopting a “serpent” strategy to strangle the communist country after President Bush warned it would suffer isolation and economic hardship unless it abandons its nuclear ambitions.

Meanwhile, a South Korean presidential envoy returned from Pyongyang and said North Korean negotiators reaffirmed the dispute about their country’s nuclear activity can only be solved through direct dialogue with the United States.

The envoy, Lim Dong-won, returned to Seoul after waiting in vain to meet North Korean leader Kim Jong Il.

Lim ruled out any quick solution to the nuclear dispute, saying it would be “a very long and gradual process.”

In Washington, Bush said in his State of the Union speech, heard in Asia early Wednesday, that the United States and other countries would not be “blackmailed” into granting concessions to North Korea by its nuclear weapons development.

North Korea did not respond directly to Bush’s speech. But soon after it was delivered, the North’s official news agency, KCNA, released a commentary saying Washington was using the nuclear dispute 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.

While calling for talks with Washington, North Korea accuses the United States of planning to launch a nuclear attack on the communist state and trying to stifle it through economic and political pressure.

Lim had hoped to meet with Northern leader Kim Jong Il to try to dissuade him from pursuing nuclear weapons development. But North Korean officials said Kim was not in the capital when Lim visited.

Lim delivered a letter from South Korean President Kim Dae-jung to the Northern leader through aides. In the letter, Kim Dae-jung urged the North to resolve suspicions that it was developing nuclear weapons and renounce nuclear ambitions in order to resume dialogue with Washington, Lim said.

Through his aide, Kim Jong Il thanked the South Korean leader “for the warm advice and recommendations, and said he would study them carefully,” Lim said.

Lim did achieve a smaller goal: He said the two sides agreed to open a cross-border railway link next month, the first since before the 1950-53 Korean War.

The neighbors also agreed to open a road on their eastern border early next month to allow Hyundai, a South Korean conglomerate, to take tourists to the North’s Diamond Mountain.

In a separate report, KCNA also quoted two North Korean newspapers — Rodong Sinmun and Minju Joson — as claiming the United States keeps 1,000 nuclear weapons in South Korea and demanding their withdrawal.

There have been no such reports in recent years. Washington traditionally maintains a “neither-confirm-nor-deny” policy about movement of its nuclear weapons.