Pat Dollard | Young Americans

VIENNA (Reuters) - U.N. nuclear investigators were winding up a four-day visit to Syria on Wednesday after checking an alleged atomic site bombed by Israel in September.

The International Atomic Energy Agency (IAEA) sent the team after receiving U.S. photos of the al-Kibar site that prompted it to put Syria on its proliferation watch list in April.

The IAEA has criticized Washington for waiting until long after the Israeli raid to brief the U.N. nuclear watchdog about its suspicions that Syria, with North Korean help, had almost completed a reactor that could have yielded plutonium for bombs.

Syria denies pursuing any clandestine nuclear project and says the Israelis hit an ordinary military structure being built in the northeastern desert near the Euphrates river.

A senior diplomat familiar with the Vienna-based IAEA said the visit by the three senior inspectors, which included a trip to al-Kibar, was going well, but declined to elaborate.

Syrian officials and state-dominated media have kept silent about the IAEA mission since it began on Sunday. The U.N. nuclear watchdog agency has declined comment.

The team led by Olli Heinonen, head of the IAEA’s global inspectorate, was due to return to Vienna later on Wednesday. The agency may request further visits to clear up months of mystery that followed the Israeli strike, diplomats have said.

Syria, an ally of Iran whose own nuclear program has been under IAEA investigation since 2003, has accused the United States of doctoring evidence in collusion with Israel, believed to be the Middle East’s only nuclear-armed power.

U.S. nuclear analysts say satellite images show the Syrians had removed debris and constructed a new building at the site destroyed by Israel — which has said nothing about the raid.

Damascus has denied concealing anything from the IAEA in possible violation of its Non-Proliferation Treaty obligations.

IAEA director Mohamed ElBaradei said last week there was no evidence that Syria, whose only declared nuclear facility is an ageing research reactor under IAEA monitoring, had the skills or fuel to run a major nuclear complex. Washington disputes this.

(Writing by Alistair Lyon)