While the US accuses Russia of supplying Syrian attack helicopters, the Pentagon defends its own $1bn deal with the same Moscow arms dealer.

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The civil and diplomatic war over Syria escalated on Wednesday with Russia accusing the US of arming anti-Assad forces, and Syria announcing the "cleansing" of rebels in Haffeh after a week of heavy fighting.

The war of words began with US Secretary of State Hillary Clinton accusing Russia of supplying the Syrian government with attack helicopters and lying about it.

"There are attack helicopters on the way from Russia to Syria which will escalate the conflict quite dramatically," Mrs Clinton told reporters on Tuesday, dismissing Russia's suggestions that its export shipments were not related to the conflict. "That's patently untrue."

Russian Foreign Minister Sergei Lavrov denied Russia violated international law by honouring a weapons contract between Syria and Moscow's state-controlled arms dealer Rosoboronexport, and he turned the table on the Americans on Wednesday accusing the US of "providing arms and weapons to the Syrian opposition that can be used in fighting against the Damascus government."

The diplomatic war

Saudi Arabia, Qatar, Syrian expatriates and wealthy individuals in Syria are also implicated in providing funds and arms directly to the Free Syrian Army rebels. Most of the provisions are light weapons but some anti-tank missiles have been procured, according to reports citing unnamed FSA sources and Syrian activists.

Moscow-based Rosoboronexport, contacted by Channel 4 News today, declined to comment on whether it was sending attack helicopters to Syria and, if so, how many. Syria is in its 15th month of a bloody conflict between rebels and Bashar al Assad's regime which has claimed at least 10,000 lives. Arms are being used in heavy fighting, most recently in Haffeh.

Rosoboronexport Deputy CEO Igor Sevastyanov told Moscow's RIA Novosti news that Russia would fulfil its long-standing contract with Syria to supply armaments he described as self-defence rather than attack weapons.

"No one can ever accuse Russia of violating the rules of armaments trade set by the international community," Mr Sevastyanov said. "There can be no talk about any violations by Russia or Rosoboronexport."

Arms supplies

Rosoboronexport, Moscow's state-controlled arms dealer, is the biggest arms supplier to Syria's government, according to Human Rights First and other reports. Russia has supplied Syria with Bastion coastal missile systems, Yakhont cruise missiles and Buk surface-to-air missile systems under a contract signed in 2007, according to research by Human Rights First.

The Syrians aren't the only country dealing with the Moscow weapons manufacturer, however.

It emerged on Tuesday that the US, which has exported military equipment to Israel and conflict zones in the past including Iraq, signed its own $1bn arms deal with Rosoboronexport in May 2011 to buy attack helicopters from Russia for use in Afghanistan.

Rosoboronexport's US deal

Rosoboronexport's contract with the US government is to supply the US defence department with the Mi series of rotory wing aircraft and parts, with an option for $550m additional purchases.

US government contract records show ongoing business between the Americans and Rosoboronexport throughout 2011 after the Syrian uprising began. The latest transaction between the US and Moscow arms supplier was in February 2012. The US has had the option of withdrawing from the $1bn contract at any point but has not done so, Reuters reported.

The US government sees no problem buying Moscow's attack helicopters for use in Afghanistan, while simultaneously condemning Moscow for allegedly supplying the attack helicopters for Syria's conflict.

The Moscow arms exporter was subject to US sanctions from 2006 to 2010 for allegedly providing Iran and Syria with equipment that could be used to develop weapons of mass destruction. But on 26 May 2011, months after the Syrian uprising began, the US changed its mind and signed its own $1bn weapons contract with Rosoboronexport.

"You could argue they shouldn't be helping those companies out...It is a matter of opinion whether that is hypocrisy," Dr Sam Perlo-Freeman, senior researcher with the Stockholm International Peace Research Institute, told Channel 4 News.

The US's 'deeply troubling' arms deal

Texas Republican Senator John Cornyn said it "makes no sense" for the US to buy arms from a Russian company enabling atrocities in Syria. Mr Cornyn has blocked confirmation of the nominee for the US army's top weapons buyer until the defence department agreed to take action.

Leon Panetta, defence secretary, confirmed the purchase of the Mi-series helicopters for the Afghan army, but the Pentagon said dealing with Rosoboronexport was the only legal way to supply the helicopters to Afghanistan.

"I remain deeply troubled." Mr Cornyn wrote in a letter to Mr Panetta. "Such actions by Rosoboronexport warrant the renewal of US sanctions against it, not a billion-dollar contract."