A senior advisor to US President Barack Obama says delivery of a Russian anti-aircraft system to Iran would have serious consequences for US ties with Moscow. Gary Samore, the White House coordinator for arms control, weapons of mass destruction proliferation and terrorism, said he thought Russia understood Washington's position and he would be surprised if Moscow shipped the S-300 anti-aircraft system that Iran has ordered. "We've made it very clear to the Russians that that would have a very significant impact on our bilateral relations," Reuters quoted Samore as saying on Tuesday. "I think the Russians understand that the consequences would be very severe," he added. Last month, a senior Russian official stated that Moscow would implement a contract with Iran which obliges Russia to deliver the advanced missile system to the Islamic Republic. "Contracts have been signed, and they are being implemented - they have not been torn up," the head of Russia's Federal Agency for Military Cooperation, Mikhail Dmitriyev, said. Russia signed a contract with Iran for the sale of at least five S-300 air defense systems in December 2005.However, Moscow has not honored the contract, possibly under pressure from Washington and Tel Aviv.