Congressional intelligence committees will be told Thursday that North Korea was helping Syria build a plutonium-producing nuclear reactor before Israeli warplanes bombed the site last September.

North Korea has long been suspected of helping Syria advance its secret nuclear program, but both countries deny it. Pyongyang says it has never spread its nuclear expertise beyond its borders.

Both House and Senate panels for the first time will be given details by the Bush administration of how North Korea was helping Syria construct a reactor similar to its facility at Yongbyon, in the west-central part of the country, according to a government official who spoke on condition of anonymity. That reactor has in the past produced a small amount of plutonium, which can be a component in nuclear weapons.

Israeli warplanes reportedly hit the site last Sept. 6, destroying what news reports then described as a nuclear facility.

When asked about the attack, a U.S. official told FOX News: "If there was an undeclared nuclear facility destroyed before it could go online... that's a good thing. And, it points to good intelligence gathering."

One senior administration official said Thursday's briefing was scheduled because the intelligence community had been deluged for months with congressional requests for information about North Korean activity in Syria and the Israeli airstrike and felt it was now time to brief lawmakers.

An official told FOX News that some committe members were already briefed.

When asked why more members were not briefed sooner, the official explained that the information from multiple sources needed to be scrubbed carefully over time to examine all "alternative explanations."

There also were concerns that the revelations, if leaked or made public too soon, could encourage opponents of the administration's attempts to negotiate an end to North Korea's nuclear weapons program. U.S. diplomats are pressing North Korea to come clean about its nuclear cooperation with Syria as part of those talks but have had little success.

At the same time, Middle East experts in the administration are worried that the timing of the briefing might upstage visits to Washington this week by Jordanian King Abdullah II and Palestinian President Mahmoud Abbas and hurt Arab-Israeli peace prospects with allegations of nefarious activity by an Arab nation with the aid of North Korea, the official said.

The official spoke on condition of anonymity to discuss elements of the classified briefing.

Moon Tae-young, the chief spokesman at the South Korean Foreign Ministry in Seoul, said his ministry would not comment on a media report on intelligence affairs.

White House National Security Council spokesman Gordon Johndroe said late Tuesday: "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."

Speculation about a possible release of information has been building, particularly in the Israeli media, for more than a week, with some reports suggesting that the briefing would include intelligence gathered by Israel and that the Israeli government had signed off on its being shared.

Another official said Thursday's presentation would be a compilation of intelligence from more than one source that has been carefully analyzed over a period of months and by its nature comes with caveats.

Under an agreement reached last year with the United States, China, Japan, South Korea and Russia, the North is required to give a full account of its nuclear programs, including whether it spread nuclear technology.

North Korea claims it gave the nuclear declaration to the U.S. in November, but U.S. officials say the North never produced a "complete and correct" declaration.

The Capitol Hill briefing also comes the same week a U.S. delegation went to North Korea to press the regime for a detailed list of its nuclear programs, the latest sticking point at international nuclear disarmament talks.

The leader of the delegation is expected to report back to Washington on Friday.

The U.S. recently has stepped back from its push for a detailed declaration addressing the North's alleged secret uranium enrichment program and nuclear cooperation with Syria. Now, the U.S. says it wants the North to simply acknowledge the concerns and then set up a system to verify the country doesn't continue such activity in the future.

President Bush defended the plans over the weekend during a meeting with new South Korean President Lee Myung-bak, saying North Korea had the burden of proof under the agreements.

The Wall Street Journal first reported Tuesday that U.S. intelligence officials would tell the committees that North Korea was helping Syria build a plutonium-fueled reactor.

FOX News' Steve Centanni, the Associated Press and Wall Street Journal contributed to this report.