"That's what we want: a credible explanation," said State Department spokesman Ian Kelly.

"We want them to open up what the IAEA is asking them to open up in terms of access to sites and access to information," Kelly said.

The International Atomic Energy Agency (IAEA) confirmed to AFP earlier Tuesday its inspectors were in Damascus after a report from the agency said it doubted Syria's initial explanation.

Asked about any possible consequences to situation, Kelly said it would "depend on the response" from Syria to the IAEA's requests.

Samples taken at the so-called Miniature Neutron Source Reactor in August 2008 confirmed the presence of "particles of anthropogenic natural uranium of a type not in Syria's declared inventory," according to the international nuclear watchdog.

The discovery of the uranium traces at the reactor raised eyebrows because the IAEA is already investigating allegations that Syria had been building an undeclared nuclear reactor in a remote desert site called Dair Alzour until Israel bombed it in 2007.