North Korea has been accused by the US of aiding Syria with the construction of a nuclear reactor that, it said "was not intended for peaceful purposes."
The site - understood to be comparable to North Korea's (now-shut down) Yongbyon reactor - was attacked by Israeli warplanes last year.
In a statement, the White House urged Syria to now "come clean" about its hidden nuclear work.
The statement was made after members of Congress had been briefed by the CIA on the situation
Syria, meanwhile, has denied the claims. According to officials, the site bombed by Israel was, in fact, a military facility which, at the time of the airstrike, had not been finished.
However, said the White House, the "cover-up" procedure performed by Syria after the Israeli bombing had taken place had strengthened its beliefs on the reactor issue.
"Until 6 September, 2007, the Syrian regime was building a covert nuclear reactor in its eastern desert capable of producing plutonium", the statement asserted, adding: "The Syrian regime must come clean before the world regarding its illicit nuclear activities."
Regarding North Korea, the same statement commented on how the US was "seriously concerned about North Korea's nuclear weapons programme and its proliferation activities".
Despite this, the White House has stressed its continued commitment to the six-nation talks, through which the pivotal nuclear-shut-down-for-aid deal was forged with North Korea last year.
The statement's release came after two days of discussions between North Korean and US officials which, it is understood, went smoothly.
The information provided by the CIA to Congress included images which, according to the US, are proof of the presence of North Koreans at the Syrian site.
The same pictures which, it is understood, were obtained by sources in Israel, showed marked similarities between the facilities in Syria and Yongbyon.
The Syrian site, however, was described by officials as not yet in operation.
"Good, clear, verifiable information" on the issue would now be needed before the US could take North Korea of its list of terrorist nations, news agency AFP quoted Pete Hoekstra, Republican Congressman, as having said.
Officials in Syria have rejected the claims, Syrian Ambassador Sami Khiyami telling the BBC that they were "ridiculous."
He added: "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."
Khiyami referred to the facility under scrutiny as an unoccupied military establishment - "nothing to do with a reactor"
Source - Armed Forces International's Middle East Correspondent
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