(Adds U.S. envoy's comment paragraphs 2-3)

SEOUL, April 24 (Reuters) - North Korea said on Thursday it made progress in talks with a visiting U.S. State Department official as the White House was expected to tell U.S. lawmakers it believes North Korea has been helping Syria build a reactor.

Sung Kim, who left Seoul for the communist state's capital on Tuesday for discussions on an international nuclear disarmament deal, said after crossing back he could not comment in detail on the meetings in the North until briefing Washington.

"We had a good visit and we had a very substantive discussion," he told reporters.

North Korea's official KCNA news agency quoted an unnamed Foreign Ministry spokesman as saying: "The negotiations proceeded in a sincere and constructive manner and progress was made there."

North Korea failed to meet a Dec. 31, 2007, deadline agreed during six-party nuclear talks to give a complete list of its fissile material and nuclear weaponry as well as answer U.S. suspicions of proliferating technology to Syria.

The spokesman said technical matters of the nuclear declaration were discussed but offered no details.

The White House has said little about the possibility of cooperation between North Korea and Syria since Israel conducted a mysterious Sept. 6 air strike on Syria that media reports said targeted a nuclear site being built with Pyongyang's help.

The Bush administration briefing with U.S. lawmakers is on Thursday, a U.S. official said. (Reporting by Jon Herskovitz; Editing by David Fogarty)

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