Angered by Russia's refusal to stop selling arms to the regime of Syrian President Bashar al-Assad, U.S. lawmakers are urging the Defense Department to halt U.S. purchases of Russian helicopters and parts for the Afghan air force.

In a letter to Defense Secretary Leon Panetta, 17 senators said that "U.S. taxpayers should not be put in a position where they are indirectly subsidizing the mass murder of Syrian civilians." The letter, dated Monday, was sent to Secretary of State Hillary Clinton as well.

But State Department spokeswoman Victoria Nuland on Tuesday defended the Russian-Afghan deal, saying that although "we obviously share the intent" of the senators' demands to persuade Russia to end its arms supply to Syria, "cutting off U.S. purchases would hurt Afghanistan's ability to defend itself."

Last May, the Defense Department signed a $375 million contract with the Russian state-controlled arms export firm Rosoboronexport to purchase 21 Mi-17 aircraft. The helicopters, developed by the Soviet Union, are used extensively around the world and Afghan pilots traditionally have been trained on them.