Russia threatened Friday to block a UN Security Council report on Iran sanctions, calling the work which said Iran has been shipping weapons to Syria "loose and sloppy."

Russia's UN ambassador Vitaly Churkin said his mission did not agree with the report and would be raising objections with the 15-nation Security Council.

"We believe that it is a loose and sloppy piece of work and we believe that there are some recommendations which our experts do not agree with at all," Churkin told reporters.

He said "problems" with the panel of experts report would be raised on the Iran sanctions committee, which diplomats said will not meet until early June.

"These are highly technical matters and we have about two or three pages of concerns and disagreement on proposals and conclusions that are made. So this is something that we intend to raise with our colleagues," Churkin said.

The report said four rounds of UN sanctions are slowing Iran's nuclear program but the Islamic Republic has breached an arms embargo by shipping weapons to Syria.

Churkin would not give details on the Russian objections. But it can block the report as the study can only be published if all nations agree.

Syria is a key Russian ally in the Middle East and some diplomats have said Russia may be trying to protect President Bashar al-Assad, who faces international criticism over his government's deadly crackdown on opposition protests.

Six of the nine violations of a UN ban on conventional arms shipments by Iran involve Syria, said the report, a copy of which was obtained by AFP.

The UN Security Council has passed four rounds of sanctions against Iran over its uranium enrichment, which Western nations say is a front for a nuclear arms drive.

The sanctions are "slowing Iran's nuclear program but are not yet having an impact on the decision calculus of its leadership with respect to halting uranium enrichment and heavy water-related activities," the experts said.

Iran is believed "to be coming close to exhausting its supply of uranium oxide." It may therefore be seeking new sources of uranium to expand its enrichment, they added.

International embargoes, assets freezes and travel bans have "clearly forced changes in the way in which Iran procures items," said the report.

"At the same time, Iran's circumvention of sanctions across all areas, in particular front companies, concealment methods in shipping, financial transactions, and the transfer of conventional arms and related materiel, is willful and continuing."

The panel of experts recommended adding the names of new individuals and companies to the UN sanction list.

"The panel notes that most reported incidents of conventional arms-related violations involve Syria, which has a long and close relationship with Iran," said the report.

On top of nine incidents listed, the experts added that it was likely other undetected arms shipments had been made.

Israel seized one Iranian shipment to the Hamas group in Gaza, which was carried out through Syria.