Iran has been using civilian aircraft to fly military personnel and large quantities of weapons across Iraqi airspace to Syria to aid president Bashar al-Assad in his attempt to crush an 18-month uprising against his government, according to a Western intelligence report seen by Reuters.

Earlier this month, US officials said they were questioning Iraq about Iranian flights in Iraqi airspace suspected of ferrying arms to Assad, a staunch Iranian ally. Today, US Senator John Kerry threatened to review US aid to Baghdad if it does not halt such overflights.

Iraq says it does not allow the passage of any weapons through its airspace. But an intelligence report obtained by Reuters says Iranian weapons have been flowing into Syria via Iraq in large quantities. Such transfers, the report says, are organized by the Islamic Revolutionary Guard Corps.

"This is part of a revised Iranian modus operandi that US officials have only recently addressed publicly, following previous statements to the contrary," said the report, a copy of which was provided by a UN diplomatic source.

"It also flies in the face of declarations by Iraqi officials," it said. "Planes are flying from Iran to Syria via Iraq on an almost daily basis, carrying IRGC (Iranian Revolutionary Guard Corps) personnel and tens of tons of weapons to arm the Syrian security forces and militias fighting against the rebels."

Although the specific charges about Iraq allowing Iran to transfer arms to Damascus are not new, the intelligence report alleges that the extent of such shipments is far greater than has been publicly acknowledged, and much more systematic, thanks to an agreement between senior Iraqi and Iranian officials.

Iraqi officials in Baghdad and New York did not have any immediate comment.

The issue of Iranian arms shipments to Syria came up repeatedly at a Senate hearing in Washington today on the nomination of Robert Beecroft as the next US ambassador to Baghdad. Mr Beecroft is currently deputy chief of mission there.

John Kerry, the Democratic chair of the Senate Foreign Relations Committee, asked Mr Beecroft what the embassy was doing to persuade the Iraqis to prevent Iran from using their airspace for flights carrying weapons to Syria. Mr Beecroft said that he and other US officials made clear to Iraq the flights must stop.

Reuters