Syria remains the top destination for Iranian arms shipments, in violation of a UN Security Council ban on weapons exports by the Islamic Republic, according to a confidential report.
The report, submitted by a panel of sanctions-monitoring experts to the Security Council's Iran sanctions committee, said the panel investigated three large illegal shipments of Iranian weapons over the past year.
Iran, like Russia, is one of Syria's few allies as it presses ahead with a 14-month-old assault on opposition forces determined to topple Syrian President Bashar al-Assad.
The report comes as Iran and the UN International Atomic Energy Agency try to narrow their differences on how to tackle concerns over Iran's atomic programme, and as Iran prepares for talks with the five permanent Security Council members and Germany in Iraq next week.
The report was leaked to the Reuters news agency hours after an article appeared in the Washington Post revealing how Syrian opposition fighters battling Assad's government are beginning to receive more, and better, weapons in an effort paid for by Gulf Arab states and co-ordinated partly by the US.
The article cited opposition activists and US and foreign officials, detailing how the administration has expanded contacts with opposition military forces to provide the Gulf states with assessments of fighters' credibility and command-and-control infrastructure, the paper reported.
"We are increasing our nonlethal assistance to the Syrian opposition, and we continue to co-ordinate our efforts with friends and allies in the region and beyond in order to have the biggest impact on what we are collectively doing," a senior US state department official told the Post.