Damascus says the facility was military, but claims it was not a nuclear one.
Washington also took its case to the UN nuclear watchdog, the International Atomic Energy Agency (IAEA), alleging that Damascus violated its obligations under the 1970 Nuclear Non-Proliferation Treaty, a US official said.
In Congress, top US officials were laying out the charges behind closed doors in a video presentation that includes photographs of the facility, a top US official said on condition of anonymity.
Another US official, who also requested not to be named, said the reactor could have produced plutonium, potentially to feed nuclear weapons, but was destroyed before it ever began to operate.
Syria denounced the charges, with its Ambassador to the United States pointedly linking the supposed US evidence to Washington's weapons-of-mass-destruction case for invading Iraq.
The allegations were certain to roil six-country diplomatic efforts to get North Korea to come clean on its nuclear and proliferation activities and abandon its atomic ambitions in return for diplomatic and economic rewards.
They could also have dramatic repercussions for Syria, an ally of US archfoe Iran, and a frequent target of fierce US criticisms over its influence in Lebanon and charges of letting Islamist fighters into Iraq.
White House officials declined to discuss the impact of the revelations on the North Korean diplomacy, or to restate President George W. Bush's repeated warnings that he viewed any such activities by North Korea as "a grave threat" with ominous "consequences."
They also sidestepped questions on whether the alleged activities needed to be included in a formal "declaration" that North Korea had been due to provide by December 31, 2007 but is still the subject of hard-fought negotiations.
Bush, who is eager for a final resolution of the nuclear crisis before he leaves office in January 2009, was not expected to comment publicly, and the White House offered only spare information.
"There are (Capitol) Hill briefings ongoing today regarding the North Korean-Syria matter that you've been reading about in the newspapers," said spokeswoman Dana Perino, who promised a formal statement later in the day.
National Security Adviser Stephen Hadley and CIA chief Michael Hayden were seen going into a joint meeting of the House and Senate Armed Services Committees, the first of several key panels expected to be briefed.
"In regards to a videotape, I'll let the intelligence community talk about that," said Perino, in reference to news reports about the centerpiece of the briefings.
A US official, requesting anonymity, said: "There are still photographs of the facility as part of the video, but it's a video presentation, like a Powerpoint presentation. It's not a video of the facility."
The New York Times and The Washington Post, had earlier cited unnamed senior US officials saying a video showing North Koreans inside the Syrian reactor would be shown at the hearings.
The Syrian reactor is said to appear to be identical in design to a North Korean reactor at Yongbyon.
Thursday's briefings come after months of questions over an Israeli airstrike in September on a mystery target widely reported to have been a fledgling nuclear site.
Before the briefings, John Rood, acting under secretary for Arms Control and International Security, briefed IAEA chief Mohamed El Baradei about the details being given Congress.
"Rood talked to El-Baradei this morning," a US official told reporters on condition of anonymity.
The United States also wanted to "have the IAEA take up the case with the Syrians that they're not complying with their (Non-Proliferation Treaty) obligations," the official added.
Source: AP
Tags: IAEA, intelligence, Israel, Mohamed El Baradei, North Korea, Nuclear, Syria, US
