The United States has accused North Korea of helping Syria build a nuclear reactor that "was not intended for peaceful purposes".
The site, said to be like one in North Korea, was bombed by Israel in 2007.
Syria must "come clean" about its secret nuclear programme, the White House said in a statement after CIA officials briefed members of Congress.
Syria has repeated denials that it has any nuclear weapons programme, or any such agreement with North Korea.
Syrian officials have said the site that was bombed by Israel in September last year was an unused military facility under construction. Building on the site had stopped some time before the air strike, the Syrians said.
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SEQUENCE OF EVENTS
6 Sept 2007: Israel bombs site in Syria
1 Oct: Syria's President Assad tells BBC site was military
24 Oct: New satellite images taken show site bulldozed clear
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In late October 2007, an independent American research organisation, the Institute for Science and International Security (ISIS), released pre- and post-strike satellite images of the site which indicated it had been bulldozed flat after the bombing.
But the White House said the "cover-up" operation that Syria carried out after the Israeli air strike reinforced its belief that the alleged reactor "was not intended for peaceful activities".
"Until 6 September, 2007, the Syrian regime was building a covert nuclear reactor in its eastern desert capable of producing plutonium," the statement said.
"The Syrian regime must come clean before the world regarding its illicit nuclear activities."
The statement added that the US had long been "seriously concerned about North Korea's nuclear weapons programme and its proliferation activities".
Images released by the CIA- footage courtesy of US Government video
But the White House insists it is committed to the on-going six-nation diplomacy, between North Korea and US, China, Japan, South Korea and Russia that led to a landmark deal with Pyongyang, in February 2007.
Ulterior motive?
North Korea agreed to give up its nuclear weapons in return for aid and its removal from a blacklist of state sponsors of terrorism. But the US has accused Pyongyang of missing the deadline to make a full nuclear declaration as promised.
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Compare before and after satellite images of the site
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The CIA briefing and statement coincides with the end of a two-day meeting between US and North Korean officials on Pyongyang's nuclear programme, which both sides say have gone well - fuelling speculation that a deal may be imminent.
The BBC's James Coomarasamy, in Washington, says the question being asked by some in the US capital is whether the reactor statement is designed to reinforce those diplomatic efforts or an attempt by some in the administration to undermine them.
Damning images?
The CIA briefings included pictures which the US says prove that North Koreans were working inside the secret site.
One of the images, which shows two men standing side by side, was said by the CIA to be of the head of the North Korean nuclear plant and the head of the Syrian atomic energy commission together in Syria.
The images - said to have been obtained by Israel - showed striking similarities between the Syrian facility and the North Korean reactor at Yongbyon, the US said.
However, the facility was not yet operational and there was no fuel for the reactor, officials said.
Israeli fears
Republican Congressman Pete Hoekstra, who attended the CIA briefing, said the US needed "good, clear, verifiable information" from the countries involved before North Korea could be removed from the terrorism blacklist, the AFP news agency quoted him as saying.
 The US says this is the North Korean and Syrian nuclear chiefs in Syria
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Mr Hoekstra criticised the Bush administration for waiting eight months to brief the intelligence committee and warned that it could jeopardise any future agreement with North Korea.
Syrian officials have denied any North Korean involvement in their country.
"These allegations are ridiculous," Syria's ambassador to the UK, Sami Khiyami, told the BBC.
"We are used to such allegations now, since the day the United States has invaded Iraq - you remember all the theatrical presentations concerning the WMDs [weapons of mass destruction] in Iraq."
Mr Khiyami said the facility was a deserted military building that had "nothing to do with a reactor".
Syria is a signatory to the Nuclear Non-Proliferation Treaty (NPT), which gives it the right to enrich its own fuel for civil nuclear power, under inspection from the International Atomic Energy Agency (IAEA).
North Korea has previously denied transferring nuclear technology to Syria.
The BBC's Katya Adler in Jerusalem says the apparent strike on the reactor, deep inside Syria, was seen by many in Israel as a sign of their military prowess.
But she says Israeli defence officials now have expressed concern over the revelation of classified data in the US.
got in their photo gallery... 
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