Russia may freeze the sale of state-of-the-art S-300 missile systems to Iran, a Moscow source was quoted by the Interfax news agency as saying Tuesday. "Such a possibility cannot be ruled out. A decision on this issue should be made at the political level, because the contract has moved outside the purely commercial framework," an unnamed source told the news agency. He emphasized that the contract was signed in 2005, but that delivery of the anti-missile systems had still not taken place. It comes a week Moscow and the US declared their willingness to bolster ties.