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. The timing of this comment might not be coincidental. The planned delivery of the S-300 to Iran has been met with fierce criticism from both Israel and the United States. The news came as the US and Russia announced efforts to improve their relations. There are indications that US President Barack Obama is reconsidering placing a controversial US missile defense system in Eastern Europe. In a letter to Russian President Dmitry Medvedev, Obama said Russia's impeding of Iran's pursuit of nuclear weapons would reduce the need for a US missile-defense system in Europe. The project is meant to counter potential missile attacks from Iran or North Korea. Moscow opposes this project, because it fears the radars could threaten its defense and be used to spy on Russia. It is possible Russia is reconsidering the sale of the S-300 system in order to please Washington and persuade the US to backtrack on the radar deployment. The S-300 is an anti-missile and anti-aircraft surface-to-air system. The most advanced version can intercept missiles and aircraft from more than 120 kilometers away, which could ward off any potential attack on Iran's nuclear facilities. Russia has existing economic and defense deals with several countries in the Middle East, including Iran. Moscow is helping Iran build the Bushehr nuclear facility.