Leaked report to UN committee by panel of sanctions-monitoring experts says Tehran violating UN Security Council ban.
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 oust Syrian President Bashar al-Assad.
The report comes as Tehran 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 nations 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 nations 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.
The panel on Iran said of the three shipments that it had investigated: "Iran has continued to defy the international community through illegal arms shipments.
"Two of these cases involved [Syria], as were the majority of cases inspected by the Panel during its previous mandate, underscoring that Syria continues to be the central party to illicit Iranian arms transfers."
The third shipment involved rockets that Britain said last year were headed for Taliban fighters in Afghanistan.
"The panel recommends the designation [blacklisting] of two entities related to these interdictions," it said.
"The report also takes note of information concerning arms shipments by Iran to other destinations."
The kinds of arms that Iran was attempting to send to Syria before the shipments were seized by Turkish authorities included assault rifles, machine guns, explosives, detonators, 60mm and 120mm mortal shells and other items, the panel said.
The most recent incident described in the report was an arms shipment discovered in a truck that Turkey seized on its border with Syria in February. Turkey announced last year that it was imposing an arms embargo on Syria.
Diplomats said that the panel's draft report may be changed by the Security Council's Iran sanctions committee before it is submitted to the council itself for consideration.
It was unclear how long it would take the committee to pass the report to the Security Council. Last year's expert panel report on Iran was never made public because Russia blocked its publication.
The report also discusses Iran's attempts to circumvent sanctions on its nuclear programme but notes that the four rounds of punitive measures the 15-nation Security Council imposed on Iran between 2006 and 2010 are having an impact.
"Sanctions are slowing Iran's procurement of some critical items required for its prohibited nuclear program," it said.
"At the same time prohibited activities continue, including uranium enrichment."
Among the items Iran has attempted to procure for its nuclear programme, the panel said, were nuclear-grade graphite, high-strength aluminum, aluminum, powder, specialised alloys, maraging steel, carbon fiber, magnets, vacuum pumps, turbines, electrical switchboards and helium gas detectors.
"The panel identifies the acquisition of high-grade carbon fibre as one of a number of critical items Iran requires for the development of more advanced centrifuges," the report said, adding that nations should be on alert for illicit attempts to acquire such items.
Iran rejects allegations by Western nations and their allies that it is secretly developing the capability to produce nuclear weapons.
It has refused to suspend its enrichment programme as demanded by the Security Council despite being hit with increasingly strong UN and various national sanctions.