"We are convinced, based on a variety of information, that North Korea assisted Syria's covert nuclear activities," Dana Perino, a White House spokeswoman, said.
"We have good reason to believe that reactor, which was damaged beyond repair on September 6 of last year, was not intended for peaceful purposes."
However, Syria's ambassador to the US on Thursday dismissed the White House's claim that North Korea had helped it build a nuclear reactor as a "ridiculous story".
Imad Moustapha told Al Jazeera that his government maintained there was no evidence of any alleged secret nuclear activity.
If there had been a nuclear reactor, there would have been security such as anti-aircraft weapons and guards there, he said.
The claims follow a briefing of US congressional officials in Washington DC by intelligence chiefs, including William Hayden, the CIA director.
Before the briefing, Syria's ambassador to the UK, Sami al-Khiyami, said that co-operation between the two countries was mainly economic and had "nothing" to do with building such a facility.
However some US legislators earlier warned that the claims could wreck vital six-party talks on North Korea's nuclear programme.
The controversy began last September, when an Israeli air raid destroyed a target in Syrian territory which some reports later said was a nuclear facility being built with North Korean help.
Syria, a signatory to the nuclear Non-Proliferation Treaty (NPT), has maintained in the past that the site was an unused military facility.
It later razed the site and built a larger building in its place.
|Israel destroyed the building in a raid in|
September last year [Reuters]
The target of Israel's raid has been veiled in secrecy, with US intelligence and government officials refusing to confirm for months that such a raid even took place.
A document released by US intelligence officials on Thursday said that the administration had concluded that the suspected reactor would have been able to produce plutonium for nuclear bombs and was "nearing operational capability in August 2007".
The US did not give Israel any "green light" to attack the Syrian site, a US official said.
Senior US intelligence officials said the suspected reactor closely resembled the Yongbyon nuclear facility in North Korea, which tested a nuclear device in October 2006.
In the briefings to congress and reporters, US officials produced before-and-after aerial photographs of the suspected reactor in eastern Syria as well as detailed interior images that they said showed key parts of its components.
One photograph showed what a US intelligence official said was a senior North Korean nuclear expert standing beside a Syrian atomic official inside Syria.
Joseph Cirincione, an expert on nuclear proliferation, told Al Jazeera on Thursday that North Korea rather than Syria was the target of the briefing, as "very few officials ... are actually worried that Syria had then or does have now a programme that would threaten the US and Israel."
"There are conservatives in the US who don't like the terms of the deal the US has worked out, they think it's a little too weak," he said.
"There are others, such as the neo-conservatives Dick Cheney, John Bolton ... who don't like any agreement and who don't want to set a precedent of negotiating with a tyrannical regime [North Korea] for fear that it would set a precedent for the US negotiating with Iran."
In addition to the US briefing, John Rood, acting under secretary for arms control and international security, briefed Mohamed El Baradei, who heads the International Atomic Energy Agency (IAEA), about the details being presented to Congress, US officials told AFP news agency.
The allegations of Syrian-North Korean nuclear co-operation come at a sensitive time for Pyongyang, which reached agreement over its nuclear weapons programme last year in six-party talks with the US, China, Japan, South Korea and Russia.
On Tuesday, a delegation of US nuclear experts travelled to North Korea for talks with senior officials on the nuclear issue and are expected to report back to Washington on Friday.