"Russia has objected to the publication of the report as an official Security Council document," a council diplomat told Reuters on condition of anonymity. Several other diplomats confirmed it.
"It's obviously an attempt to protect (Syrian President) Bashar al-Assad," who is coming under increasing international pressure over his violent crackdown on anti-government protesters, another council diplomat said.
The confidential report, obtained by Reuters, said most of Iran's breaches of the embargo have been deliveries of weapons to Syria, which Western diplomats say were to be passed on to Lebanese and Palestinian militants.
The report by the UN Security Council's so-called Panel of Experts, a newly formed committee that reports on compliance with four rounds of UN sanctions imposed on Iran for refusing to halt its nuclear enrichment program, also says Tehran flouts the sanctions as it continues to develop its atomic program.
Diplomats said Russia offered a procedural justification for objecting to the publication of the report -- it should first be discussed by the Security Council's Iran sanctions committee before being released to the public.
"Eventually they'll have to give in but we don't know how long it will take," a diplomat said.
Russia is able to block the Iran report because decisions about such reports are made by consensus among the 15 members of the Security Council.
Russia's decision to block the report comes as diplomats said Britain and France were attempting to revive plans to have the Security Council condemn Syria for its crackdown against demonstrators.
A previous attempt failed after Russia, China and India objected to the proposed condemnation.
A spokesman for Russia's UN mission did not immediately respond to a request for comment.
China has prevented the publication of similar UN expert panel reports on North Korea and Sudan, two countries that Beijing routinely tries to shield from Security Council criticism, for as long as half a year.
Russia has long acted as a kind of protector for Iran on the council, working hard to dilute the four sanctions resolutions on Iran between 2006 and 2010 before voting in favor of them. It also has close commercial ties to the Islamic Republic.
A senior Russian diplomat was quoted by Russian news agencies as saying Thursday that Iran's Russian-built Bushehr nuclear power plant will be fully operational within weeks.
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