WASHINGTON (Reuters) - The Bush administration plans to brief U.S. lawmakers behind closed doors on Thursday about North Korea’s suspected nuclear cooperation with Syria, congressional sources said on Tuesday.

Sung Kim, a U.S. State Department official in charges of Korean affairs, reacts as he listens to reporters' questions upon his arrival at the Incheon international airport, west of Seoul, April 21, 2008. REUTERS/Jo Yong-Hak

The White House has said little in public about the issue since Israel conducted a mysterious September 6 air strike inside Syria that U.S. media reports said was directed at a nuclear site that may have been built with North Korean assistance.

While a handful of lawmakers were briefed on the issue last year, the decision to widen the circle substantially comes as Washington appears closer to a deal for North Korea to provide a long overdue declaration of its nuclear programs.

Once the poor, communist state has produced the declaration, the United States is expected to ease sanctions on Pyongyang that flow from its presence on the U.S. list of state sponsors of terrorism and the U.S. Trading With the Enemy Act.

A senior congressional aide and a former Bush administration North Korea specialist said they believed the briefings were designed to persuade members of Congress that removing those sanctions was justified.

Congressional sources said the briefings would be for members of the House of Representatives and Senate committees that deal with armed services, foreign affairs and intelligence.

Spokesmen for the White House and for the office of the Director of National Intelligence declined comment.

“The administration routinely keeps appropriate members of Congress informed of national security and intelligence matters, but I’m going to decline to comment on any specific briefings,” said National Security Council spokesman Gordon Johndroe.

Asked why the briefings were happening now, the senior congressional aide said, “Because they are about to lift sanctions ... and they want to convince members that we have enough clarity from what the North Koreans have acknowledged to us, and what we have learned through our own methods, to proceed with confidence.”

COMPROMISE WITH NORTH KOREA

“Otherwise, they are just setting themselves up for legislative obstructionism on the lifting of the sanctions,” the aide added.

Michael Green, a former National Security Council official now at the CSIS think tank, said a compromise the United States and North Korea reached recently in Singapore on the declaration had run into congressional resistance.

Under that compromise, North Korea would produce a declaration of its plutonium-related program but would address suspicions on nuclear proliferation with Syria and on its suspected uranium enrichment program in a different way.

According to people familiar with the plan, Washington would put forward its concerns about North Korea’s suspected uranium enrichment program and nuclear proliferation and North Korea would then “acknowledge the U.S. concerns.”

“The news about the Singapore agreement is becoming public and some members on the Hill ... are realizing that the Singapore agreement does not cover proliferation with Syria or the uranium enrichment program,” Green said.

“The administration is now being asked, in order to (win) congressional support for lifting sanctions ... to explain more details about the parts that are no longer covered (by the declaration),” he added. “As Congress looks at the details, it might make it much harder (to get that support).”

North Korea’s failure to produce the declaration has bogged down a 2005 multilateral deal under which it committed to abandon all nuclear weapons and programs.

The so-called six-party agreement was hammered out among the two Koreas, China, Japan, Russia and the United States.

The declaration has been held up partly because of Pyongyang’s reluctance to discuss any transfer of nuclear technology to other countries, notably Syria, as well as to account for its suspected pursuit of uranium enrichment.

Uranium enrichment could provide North Korea with a second way to produce fissile material for nuclear weapons in addition to its plutonium-based program, which it used to test an atomic device in October 2006.