US intelligence officials have been presenting evidence to Congress they say shows that North Korea has been providing nuclear assistance to Syria.
The CIA says it has pictures proving that North Koreans were working inside a secret Syrian reactor, according to reports in the US media.
The reactor was destroyed by an Israeli airstrike last year, the reports say.
Syria has repeated denials that it has any nuclear weapons programme, or any such agreement with North Korea.
The claims follows an unexplained air strike by Israel last September on a target inside Syria.
The images - said to have been obtained by Israel - showed striking similarities between the Syrian facility and the North Korean reactor at Yongbyon, reports said.
However, the facility was not yet operational and there was no fuel for the reactor, officials said.
Republican Congressman Pete Hoekstra, who attended the CIA briefing, said there was a "a serious proliferation issue, both for the Middle East and the countries that may be involved in Asia".
The US needed "good, clear, verifiable information" from the countries involved before North Korea could be removed from a blacklist of state sponsors of terrorism, the AFP news agency quoted him as saying.
North Korea agreed to give up its nuclear weapons in return for removal from the blacklist and aid as part of a six-nation deal agreed between North Korea and US, China, Japan, South Korea and Russia in February 2007.
The US has accused Pyongyang of missing the deadline to make a full nuclear declaration as promised.
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.
The White House has not commented on the Syrian reactor reports, but Secretary of Defense Robert Gates said information on the issue could be made public "soon".
Syria's ambassador to the UN, Bashar Jaafari, denied the links.
"There was no Syria-North Korea co-operation whatsoever in Syria. We deny these rumours," he said.
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.
The CIA briefing 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.
In the landmark deal reached in 2007, Pyongyang agreed to close its main reactor and divulge the full extent of its nuclear programme by December.
While it is taking steps to close its Yongbyon reactor, it has yet to produce a declaration of nuclear activities to the international community's satisfaction.
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Tags: Syria, CIA, Nuclear reactor, North korea, nuclear weapons, middle east, Israel, IDF, raid, international terrorism, war on terror, Kim jong IL
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