Japan however warned that if proven, the US allegations would be a blow to the North Korean talks.
"If North Korea supported Syria's nuclear activities, it would be a big problem," Nobutaka Machimura, the chief cabinet secretary, told a news conference in Tokyo.
Japan has been a key player in the six-party talks process and the most vocal advocate for a firmer line on North Korea.
It has refused to provide any aid under the disarmament deal due to a dispute with North Korea over its past kidnappings of Japanese nationals.
|An image of what the US alleges is |
the Syrian nuclear reactor [AFP]
Syria, whose officials have been briefed on the US allegations, has rejected the American claim while North Korea has in the past repeatedly denied proliferation.
Speaking to Al Jazeera, Richard Broinowski, Australia's former ambassador to South Korea cast doubt on the US claims, given the US intelligence community's failure on Iraqi weapons on mass destruction.
He said the seven months it has taken the US to come up with its alleged evidence was also a major cause for concern.
"It's very difficult to give these agencies credibility, especially when there's been such a long delay," Broinowski told Al Jazeera.
"There's a good deal of a paranoia here and we should really be careful in our terms and about what's really happening in Syria."
Dana Perino, the White House spokeswoman, said Bush administration remained committed to the six-party talks.
But she added the US would work with China, Japan, Russia and the two Koreas to create "a rigorous verification mechanism to ensure that such conduct and other nuclear activities have ceased".
The US accusations came a day after North Korea gave an upbeat assessment after a long stalemate in implementing the aid-for-disarmament deal.
Pyongyang said it had made progress in two days of talks with a US team visiting the North to discuss the declaration of all its nuclear activities.
US media have suggested the timing of the Syria allegations could be an effort by conservatives within the Bush administration to pour cold water on the six-nation deal as being too weak.
A six-party deal last year offered North Korea energy aid and major diplomatic and security benefits in return for full denuclearisation and disclosure of its programmes.
But the talks have stalled over the requirement that the North declare all its nuclear activities by the end of 2007. The North said it delivered the documentation last November but the US said it was incomplete, particularly in terms of information related to the North's alleged proliferation of nuclear technology..
The US admitted last week that it was scaling back demands on North Korea.
The US chief negotiator Christopher Hill, in talks with his North Korean counterpart this month, reportedly offered a face-saving gesture to allow Pyongyang to keep private its reply to certain US allegations, including over co-operation with Syria.
In return, Pyongyang would reportedly not protest against those allegations.