US senators are urging the Pentagon to cancel a contract with a Russian company approaching $1 billion to buy helicopters for Afghanistan, voicing outrage over Moscow's arming of Syria.

"US taxpayers should not be put in a position where they are indirectly subsidizing the mass murder of Syrian civilians," 17 senators across party lines wrote Monday in a letter to Secretary of Defense Leon Panetta.

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 $550 million worth more, according to the letter.

Russia has refused to stop arms shipments to Syria and has offered diplomatic support to President Bashar al-Assad as he puts down a year-long revolt that activists say has killed more than 8,500 people, mostly civilians.

The senators voiced alarm at reports that Rosoboronexport has shipped arms to Syria and that Syrian forces used Russian weapons in opposition stronghold Homs.

Activists recently said the throats of 47 women and children were slit in a massacre in Homs, following a month-long bombardment of the rebellious Baba Amr neighborhood where 700 people were said to have died.

"We urge you to use all available leverage to press Russia and Russian entities to end their support of the Assad regime, and that includes ending all (Department of Defense) business dealings with Rosoboronexport," the senators wrote to Panetta.

The letter's signatories included Dick Durbin, the number-two senator from President Barack Obama's Democratic Party, and Jon Kyl, the number-two senator from the rival Republican Party.

State Department spokeswoman Victoria Nuland did not take a position on the senators' letter but said the contract would upgrade the Russian-made fleet that forms the backbone of Afghanistan's fledgling military.

"We obviously share the intent, which is to persuade Russia to end its arms supply to Syria," Nuland told reporters.

But she said if the contract were canceled, "it would seriously hurt our effort to get the Afghans increasingly into the lead of their own security."

President Barack Obama hopes that Afghan forces can take care of their own security to allow US forces to leave by the end of 2014, ending an increasingly unpopular war launched more than a decade ago after the September 11, 2001 attacks.

The task has become even more urgent amid outrage over a US soldier's massacre of 16 Afghan villagers.