Russia and the U.S. signed a contract in May 2011 for the sale of 21 Mil Mi-17 helicopters for $375 million to be paid by 2016, with a possible extension worth another $550 million.
The helicopters are intended for use by Afghan security forces, who have more experience flying Russian helicopters than American ones.
The group of 17 senators, including both Republicans and Democrats, sent a letter to U.S. Defense Secretary Leon Pannetta calling for the contract to be cancelled due to their "grave concern" about doing business with Russian defense contractors, since Russia also sells arms to the regime of Bashar Assad in Syria.
"In the face of the international community's concern [Russia] is continuing to enable the Assad regime with the arms it needs to slaughter innocent men, women and children in Syria," they wrote.
State Duma deputies have expressed confusion about the letter.
"A number of senators have recently made statements that seem strange at best. Possibly this statement is one of them," said Irina Yarovaya, head of the Duma's security and anti-corruption committee, RIA-Novosti reported.
Russia has strongly defended its arms sales to Syria in the past.
"We are not breaking any international agreements, no Security Council resolutions. We are trading with Syria only in a way that is allowed by international law," Russian Foreign Minister Sergei Lavrov said in January.