The Pentagon on Tuesday defended plans to buy attack helicopters from a Russian arms firm for the Afghan government even though the same company has supplied weapons to Syria's regime.

US senators have voiced dismay at the deal with Rosoboronexport, but defense officials said the contract with the firm was the only way to bolster Afghanistan's fleet of Russian-made choppers.

"We're not buying helicopters for the Syrian regime. We're buying helicopters in support of the Afghan Air Force," press secretary George Little told reporters.

Senator John Cornyn, in a letter to Defense Secretary Leon Panetta on Monday, expressed outrage at the purchase of Mi-17 helicopters for Afghanistan from Rosoboronexport.

"I remain deeply troubled that the DoD (Department of Defense) would knowingly do business with a firm that has enabled mass atrocities in Syria.

"Such actions by Rosoboronexport warrant the renewal of US sanctions against it, not a billion-dollar DoD contract," Cornyn wrote.

The United States plans to buy 21 Mi-17 helicopters for the Afghan military from Russia's Rosoboronexport by 2016. The contract totals $375 million by 2016, with an option to buy additional aircraft worth $550 million.

Pentagon spokesman Captain John Kirby said: "This particular contract is the only method legally available to the department to provide those aircraft and, just as importantly, safe and reliable spare parts and equipment to support those aircraft for the Afghan military."

Russia has refused to stop arms shipments to Syria and has offered diplomatic support to President Bashar al-Assad as he seeks to crush a year-long revolt.

James Miller, under secretary of defense for policy, has acknowledged in a letter to lawmakers that "Rosoboronexport continues to supply weapons and ammunition to the Assad regime and... there is evidence that some of these arms are being used by Syrian forces against Syria's civilian population."

US Secretary of State Hillary Clinton on Tuesday accused Russia of sending more attack helicopters to Syria and said Moscow was lying about its arms shipments.

The Pentagon said it could not confirm that any new Russian arms shipments had arrived in Syria.

Defense officials also denied that US reliance on Russian territory for a northern supply route into Afghanistan could make Washington reluctant to push Moscow too hard over the violence in Syria.

"I don't think we're linking the two (issues)," Kirby said.

"Russia has been extraordinarily helpful, and we're grateful for the assistance that they've offered with respect to logistics routes in and out of northern Afghanistan, but we're not linking the two," he said.