WASHINGTON (AP) - A group of senators is urging President Barack Obama to punish Moscow's incursion into the Crimean Peninsula by terminating the remainder of a billion-dollar contract to buy helicopters from Russia's arms export agency.

WASHINGTON (AP) A group of senators is urging President Barack Obama to punish Moscow's incursion into the Crimean Peninsula by terminating the remainder of a billion-dollar contract to buy helicopters from Russia's arms export agency.

In a letter sent to Obama Thursday, the lawmakers also say the U.S. should impose sanctions banning any future dealings with Rosoboronexport. They describe the export agency as unsavory because it has supplied Syrian President Bashar Assad's military forces with arms and ammunition.

The Pentagon is buying the Mi-17 transport helicopters for Afghanistan's national security forces.

U.S. military officials have defended the contract with Rosoboronexport for 63 helicopters. They've maintained the Mi-17 is ideally suited for the Afghans, who are rebuilding their air force and need a reliable and easy-to-operate helicopter for transporting troops throughout the country.