Washington's main envoy on the North Korean nuclear issue is in Pyongyang, aiming to convince leaders there not to reverse promises they made last year. As VOA Seoul Correspondent Kurt Achin reports, if the talks fail, the stage could be set for the North to produce more material that can be used for nuclear weapons.

U.S. Assistant Secretary of State Christopher Hill made the short drive Wednesday from Seoul to South Korea's heavily armed border with North Korea, headed for a rare visit to Pyongyang.

Hill has been the point man in six-nation negotiations aimed at getting rid of North Korea's nuclear weapons capabilities. He says Pyongyang's recent moves to eject international inspectors and resume activity at its main nuclear reactor have dealt a serious blow to the process.

"What they've been doing obviously goes counter the spirit of what we've been trying to accomplish... so I would say we're in a very difficult, very tough phase of the negotiation," he said.

Before leaving the South, Hill told reporters he wants to persuade North Korea to fulfill its promise to set up verification for the nuclear declaration it made earlier this year. American and South Korean officials say the North offered clear verbal assurances at Beijing talks, last year, it would agree to a verification system in line with usual international practices - such as surprise visits by international inspectors to nuclear sites.

Hill says he understands the North is reluctant to follow through, but says the process cannot move forward without a verification agreement.

"It's not to say that we're going to have anything verified. The verification will actually take place later on," he said. "But we need to know what the rules of the road are for verification."

In August, President Bush cited the North's delay on a verification agreement when he decided not to remove North Korea from a State Department list of nations suspected of sponsoring terrorism. North Korea has maintained the verification issue is separate from the American promise to remove it from the terror list.

South Korea's Yonhap news agency quotes an unnamed South Korean government source as saying there are signs of activity near the site of North Korea's 2006 underground nuclear test. The source in the report speculates North Korea may be trying to repair the site - possibly for a followup test - and says the South is monitoring the activity closely.