UN investigators have found traces of uranium at a Syrian site that Washington says was a secret nuclear reactor almost built before Israel bombed the target last year.

Diplomats said yesterday the minute uranium particles turned up in some environmental swipe samples UN inspectors took at the site in a visit last June.

The finding was not enough to draw conclusions but raised concerns requiring clarification.

The International Atomic Energy Agency (IAEA) and Syria had no immediate comment.

However, word of the finding leaked hours after agency officials confirmed Director Mohamed El Baradei was preparing a formal report on Syria for the first time.

Moreover, Syria has been made an official agenda item at the year-end November 27-28 meeting of the UN watchdog's 35-nation board of governors, unlike previously when IAEA officials said initial inquiries were inconclusive.

Syria denies US intelligence alleging it was building a reactor with North Korean expertise meant to make plutonium, the main atomic bomb ingredient reprocessed from spent uranium fuel.

Damascus says the unverified intelligence was fabricated and Washington has no credibility in the field after using bogus evidence of an Iraqi doomsday arms programme to justify the 2003 invasion that ousted Saddam Hussein and devastated the country.

El Baradei told an IAEA board meeting in September that preliminary findings from test samples taken by inspectors granted a visit in June to the desert location hit by Israel bore no traces of atomic activity.

Diplomats accredited to the Vienna-based nuclear watchdog said some samples showed contamination with minute amounts of a uranium compound.

"It isn't enough to conclude or prove what the Syrians were doing but the IAEA has concluded this requires further investigation," said one diplomat accredited to the agency. "It was a man-made component, not natural (ore).

There is no sign there was already nuclear activity there," another diplomat said.