The White House says North Korea's secret work on a nuclear reactor with Syria was "a dangerous and potentially destabilising development for the world".
It said the episode raises doubts about Pyongyang's intention to carry through with a promised disclosure of its nuclear activities.
Seven months after Israel bombed the reactor, the White House broke its silence and said North Korea helped Syria's secret nuclear programme, and the destroyed facility was not intended for "peaceful purposes".
Top US intelligence officials who briefed reporters said they had high confidence in the judgment that North Korea had aided Syria with its nuclear programme and the intention was to produce plutonium.
But they claimed only low confidence for the conclusion that it was meant for weapons development, in part because there was no reprocessing facility at the site - something that would be needed to extract plutonium from spent reactor fuel for use in a bomb.
The Bush administration's disclosure could undermine six-party negotiations to try to resolve the nuclear standoff with North Korea.
The White House issued a two-page statement after politicians were given details about the reactor in a series of briefings on Capitol Hill that included a video presentation of intelligence information the administration contends establishes a strong link between North Korea's nuclear programme and the bombed Syrian site.
The briefing also included still photographs that showed a strong resemblance between specific features of the plant and one near Yongbyon in North Korea.
Democratic senator Joseph Biden, chairman of the Senate Foreign Relations Committee, said the revelations make clear that any deal to eliminate North Korea's nuclear programs must also stop its proliferation activities and include vigorous verification.
Still, he said the information in the briefings was not a cause to end the talks.
"To the contrary, it underscores the need for pursuing the talks, which remain our best chance to convince North Korea to abandon its pursuit of nuclear weapons and to stop proliferation," Mr Biden said.
The White House said the International Atomic Energy Agency, the UN nuclear watchdog, also was being briefed on the intelligence.
While calling North Korea's nuclear assistance to Syria a "dangerous manifestation" of Pyongyang's nuclear weapons programme and its proliferation activities, the White House said it remained committed to the talks.
The United States became aware North Korea was helping Syria with a nuclear project in 2003, said intelligence officials.
The critical intelligence that cemented that conclusion, they said, came last year: dozens of photographs taken from ground level over a period of time, showing the construction inside and outside the building.
The Israeli strike on September 6, 2007, ripped open the structure and revealed even more evidence to spy satellites: reinforced concrete walls that echoed the design of the Yongbyon reactor. After the attack, Syria erected a new building over the site.
"This cover-up only served to reinforce our confidence that this reactor was not intended for peaceful activities," White House press secretary Dana Perino said.
"The Syrian regime must come clean before the world regarding its illicit nuclear activities."
The Syrian reactor was within weeks or months of being functional when Israeli jets destroyed it, a top US official said.
The official said the facility was mostly completed but still had needed significant testing before it could have been declared operational.
No uranium, which is needed to fuel a reactor, was evident at the site, a remote area of eastern Syria along the Euphrates River.
The US official said the reactor was similar in design to a North Korean reactor at Yongbyon, which has in the past produced small amounts of plutonium, the material needed to make nuclear weapons.
Syria has maintained in the past that the site was an unused military facility.
Yesterday, the Syrian Embassy said it "regrets and denounces the campaign of false allegations that the current United States administration continually launches against Syria claiming the presence of nuclear activity".
The statement said Syria denied the allegations and viewed the administration's assertions as aimed primarily at misguiding Congress and international public opinion "in order to justify the Israeli raid in September of 2007, which the current US administration may have helped execute".
"It has become obvious that this manoeuvre on the part of this administration comes within the framework of the North Korean nuclear negotiations," the statement said.
And in an apparent reference to pre-war claims by the Bush administration that Iraq had possessed weapons of mass destruction, the statement concluded: "The Syrian government hopes that the international community and the American public, particularly, will be more cautious and aware this time around in facing such unfounded allegations."
The White House also used its statement as an opportunity to denounce the nuclear activities of Iran, which it says is a threat to the stability of the Middle East.
While Washington was awash in condemnation of North Korea's proliferation activities, the communist regime is expressing optimism about current six-party negotiations with the United States, China, South Korea, Russia and Japan. The optimism over the talks is raising hopes of breaking the impasse that has deadlocked arms negotiations.
North Korea's Foreign Ministry said yesterday that it had discussed technical matters with the Americans for moving forward on that and other agreements from the arms talks. "The negotiations proceeded in a sincere and constructive manner and progress was made," the ministry said in a statement.
As part of that process, the North is required to submit a "declaration" detailing its programmes and proliferation activity, but the talks are stalled over Pyongyang's refusal to publicly admit the Syria connection.
However, officials say the North Koreans are willing to accept international "concern" about unspecified proliferation.
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