UN nuclear officials have inspected the remote site in the Syrian desert which the US alleges was a covert nuclear plant, a diplomat told AFP on Tuesday.

The diplomat, who spoke on condition of anonymity, gave no further details.

But experts were doubtful that Syria would be able to claim its innocence even if the International Atomic Energy Agency inspectors found nothing suspicious at the site.

"If the inspectors don't find anything, it won't prove Syria's innocence, of course," said Mark Fitzpatrick, non-proliferation expert at the International Institute of Stratetic Studies.

The IAEA's deputy director general, Olli Heinonen, flew to Syria Sunday for a three-day trip to investigate allegations that a mysterious site bombed by Israel last year had been a covert nuclear nuclear reactor nearing completion.

Heinonen and two other experts had now reached the site, at Al-Kibar in a remote desert area of northeastern Syria on the Euphrates River, and inspected it, the diplomat said.

Washington claims Al-Kibar, razed to the ground by Israeli planes in September, was a nuclear facility built with North Korean help and close to becoming operational. It has provided intelligence and photographic evidence to support its claims.

Syria has denied the allegations as "ridiculous," saying the edifice was simply a disused military building.

The problem is, Damascus has wiped clean the site, making the IAEA inspection more difficult, adding to suspicion within elements of the international community about the exact nature of the site.

With the IAEA team scheduled to be back in Vienna on Wednesday evening, they are unlikely to have had either the time or the equipment to draw any definitive conclusions.

Observers expect the trip to serve as merely the start of a long investigation, similar to the IAEA's five-year probe into Iran's disputed nuclear activities.

US news reports and diplomats close to the IAEA have said that the nuclear watchdog is interested in two or three other sites used to store the debris of the destroyed building. But Syrian President Bashar al-Assad has said "talking about other sites is not within the purview of the agreement" with the nuclear watchdog.

"The inspectors will not be able to see much at all or do any excavation that would uncover the destroyed reactor foundations," Fitzgerald of London-based IISS told AFP.

"Their best tool is environmental sampling that would show the presence of uranium particles, but since there apparently was no fuel at the reactor, there should not be any uranium particles, anyway," the expert pointed out.

"The IAEA in the past has been able to find things that their hosts did not expect, as in North Korea in the early 1990s. If they are allowed to explore the Al-Kibar site beyond the area that has been bulldozed, they might try to find particles of graphite. But I expect they will be kept tightly confined," Fitzpatrick said.

The IAEA's number two Heinonen was characteristically tight-lipped about what he expected to find at Al-Kibar.

"What will be waiting there, we will see when we get there," he told journalists before boarding the plane on Sunday.

Heinonen and his team will submit their findings from the visit to the IAEA's next regular board meeting in September.

Syria, too, was coy about the visit, with no acknowledgement in the state-owned media that the inspectors were even in the country on Monday.

President al-Assad accused the US of fabricating evidence as part of a campaign to ratchet pressure on Damascus, which Washington accuses of supporting terrorism along with its key regional ally, Iran.

The German newsmagazine Der Spiegel, in its latest edition, said Damascus and Pyongyang had been trying to help Iran to develop its controversial nuclear programme through the construction of Al-Kibar.

Quoting German secret service reports, the magazine said Al-Kibar was to have been used as a temporary site for Iran to develop a nuclear bomb until it was able to do so on its own territory.

Meanwhile, IAEA chief Mohamed ElBaradei has said that the IAEA has "no evidence that Syria has the human resources that would allow it to carry out a large nuclear programme."