SEOUL U.S. diplomat Chris Hill arrived in Seoul on Tuesday ahead of a visit to North Korea to try to save a crumbling disarmament-for-aid deal and prevent Pyongyang from rebuilding its aging nuclear plant.
North Korea has threatened to restore its nuclear plant -- frozen and being disabled under the deal -- that makes bomb-grade plutonium, in anger at not being dropped from a U.S. terrorism blacklist and by Washington's verification demands.
"We need to know what the rules of the road are for verification," Assistant Secretary of State Christopher Hill told reporters.
Hill said he expects verification to include ways to check on U.S. suspicions that North Korea had a secret program to enrich uranium for weapons, giving it another path to make an atom bomb.
"That is in the overall verification package," Hill told reporters after meeting South Korea's nuclear envoy Kim Sook.
Kim said: "We expect North Korea to agree to a verification protocol soon."
Hill is set to travel by road to North Korea on Wednesday on a journey of about three hours that takes him across the heavily armed border. He expects to meet the North's nuclear envoy but did not say when he planned to return.
The International Atomic Energy Agency said last week that the North was expelling U.N. monitors from its Soviet-era nuclear plant and plans to start reactivating it in days, rolling back a disarmament-for-aid deal and putting pressure on Washington.
"What they have been doing, obviously, goes against the spirit of what we have been trying to accomplish," Hill said.
North Korea started to disable its Yongbyon plant last November as a part of the deal it reached with China, Japan, Russia, South Korea and the United States.
Experts have said most of the disablement steps that were aimed at taking about a year to reverse have been completed and North Korea, which tested a nuclear device in October 2006, cannot easily get back into the plutonium producing business.
The North has balked over U.S. demands about verification, fearing it to be too intrusive. Washington countered by making clear it would only remove Pyongyang from its terrorism list of once the North agreed to a "robust" mechanism.
"If Hill can offer a more flexible verification protocol, then there is room for compromise," said Paik Hak-soon, an expert on North Korea at the South's Sejong Institute.
(Additional reporting by Kim Junghyun; Editing by Jeremy Laurence)