Vienna  - The International Atomic Energy Agency (IAEA) has detected more uranium particles and other, possibly nuclear-related material in samples from an alleged Syrian reactor site, an IAEA report and a senior United Nations official said Friday.

The Vienna-based agency said in the report that Syria needed to do more to clear up allegations that the site bombed by the Israeli air force in 2007 was a reactor.

The IAEA reported detection of 80 particles of man-made uranium in samples taken since June at the al-Kibar site, said a senior UN official, speaking on condition of anonymity. Previous research through November had only discovered half that amount.

"It's clear that this is a significant finding," said the official. "We have found nuclear material at the site, and Syria has to explain where that material came from."

The report countered Syria's claim that the uranium originated from Israeli munitions dropped at the al-Kibar site in the desert.

"The agency's current assessment is that there is a low probability that the uranium was introduced by the use of missiles," IAEA chief Mohamed ElBaradei wrote in the report, citing chemical analyses.

The United States has alleged the installation was a nuclear reactor geared toward producing plutonium. Syria insists that Israel hit a conventional military facility.

In addition to the uranium, which is used as fuel in some types of reactors, the IAEA has detected traces of graphite, a material that can serve as a construction element in the reactor core.

"We are sure it's man-made," the UN official said about the graphite.

This finding was not included in the report as the IAEA had not yet determined if the material was of the type used in nuclear installations, he said.

So far, Syria has denied inspectors access to additional sites, despite IAEA requests.

"These measures, together with the sampling of destroyed and salvaged equipment and debris, are essential for the agency to complete its assessment," ElBaradei wrote.

The restricted report was sent to IAEA member countries, which are expected to discuss it at an upcoming meeting of the IAEA governing board, scheduled to start March 2.

US State Department spokesman Gordon Duguid said in a statement that Washington expects the IAEA meeting "to address the mounting evidence and ongoing concerns related to Syria's clandestine nuclear activities," as noted in the latest report.

"We fully support the IAEA in its investigation and urge the international community to continue insisting that Syria comply with its IAEA obligations and cooperate fully with the IAEA without delay," Duguid said.

"In particular, it is imperative that Syria fully cooperate with the IAEA by granting the IAEA unfettered access to any site or information needed for the investigation." (dpa)