(Adds State Department comment on Yongbyon, paragraphs 5-6)

By Jon Herskovitz and Jack Kim

SEOUL, Oct 3 (Reuters) - A senior U.S. envoy ended a trip to North Korea on Friday aimed at saving a crumbling disarmament deal, calling the talks substantive but declining to say if he swayed the secretive state to give up plans to restart its nuclear plant.

Assistant Secretary of State Christopher Hill had been in Pyongyang since Wednesday seeking a deal that would allow monitors into North Korea to check claims it made about its nuclear program in exchange for better trading ties and better standing in the international community.

"I don't want to talk about progress," Hill told reporters in Seoul, South Korea's capital, saying he must first brief U.S. officials and other countries before releasing details.

"I don't want to say I'm satisfied," Hill said, adding he had lengthy and substantive discussions about a verification system.

In Washington, State Department spokesman Robert Wood said North Korea did not appear to halt efforts to restart its Soviet-era Yongbyon nuclear plant during Hill's visit and that equipment was still being taken out of storage and "returned to their original locations."

"The North Koreans continue to take some steps to reverse disablement of some of the Yongbyon facilities," Wood told reporters, refusing to say where he got this information. "I don't know what their actual intent is."

Ahead of the discussions, Paik Hak-soon, an expert on North Korea at South Korea's Sejong Institute, said progress could be made if Hill offered a flexible plan to inspect the North's nuclear facilities.

But Washington said it was offering no new concessions. Hill insisted the North must allow inspectors to check U.S. suspicions it had a secret program to enrich uranium for weapons, which would give it another path to make a nuclear bomb.

South Korea's chief nuclear envoy, Kim Sook, said after meeting Hill that there could be a new round of six-way talks to discuss what had happened in Pyongyang.

The nuclear agreement North Korea struck with the United States, China, Russia, Japan and South Korea in February 2007 seemed in peril after Pyongyang vowed last month to rebuild the aging Yongbyon plant, its source for weapons-grade plutonium, in anger at not being removed from a U.S. terrorism blacklist.

Washington said it would take the North off the terrorism list, bringing economic and diplomatic benefits, once a system had been agreed to verify Pyongyang's nuclear claims.

MONITORS EXPELLED

In late September, Pyongyang ordered the expulsion of U.N. monitors from Yongbyon and said it planned to start reactivating the plant within days.

Hill said he told the North Koreans that the move would be of great concern but that he had no information on any further steps Pyongyang might have taken to restore Yongbyon.

The energy-starved North started to disable Yongbyon last November under the deal. If it backs away, it stands to lose about half a million tonnes of heavy fuel oil, or aid of equal value, that had been pledged for previous progress it made.

Pyongyang has been sending out mixed signals over the past few weeks. It fanned hopes of backing off its threats by inviting Hill to visit and holding its first direct talks with South Korea in almost a year.

But South Korean government officials have told local media the North may be looking to ratchet up tensions by making moves around its missile launch site and the spot where it carried out its first and only nuclear test two years ago.

Adding to the level of uncertainty is a huge question mark over the health of the North's supreme leader, Kim Jong-il.

U.S. and South Korean officials said he may have suffered a stroke in August, raising questions about succession in Asia's only communist dynasty and about who is making the decisions regarding its nuclear program.

On Thursday, an official newspaper referred to Kim's health for the first time since the reports of his stroke, saying he had a tiring summer inspecting all parts of his country.

"While everyone else in the world is busy vacationing, we saw a flow of news sent into the universe on Kim's endless on-site inspections through a long and rough journey," it said. (Additional reporting by Rhee So-eui in Seoul and Sue Pleming in Washington; Editing by John O'Callaghan)

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