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In June, Russia joined the United States and others in backing the latest round of sanctions against Iran over its uranium enrichment program, which the West suspects of being a cover for a nuclear weapons bid.

In another move to meet U.S. concerns, Mr. Medvedev shelved a deal to provide Iran with sophisticated S-300 air defense missiles.

The Kremlin also has offered greater assistance to the U.S.-led campaign in Afghanistan, allowing the United States and its NATO allies to ferry supplies across its territory as routes via Pakistan have become increasingly unsafe — a policy that also may be reviewed if the arms pact fails to get the support of the U.S. Senate.

Asked whether Moscow could rethink its support for the United States in Iran and Afghanistan, Foreign Minister Sergey Lavrov told the Interfax news agency this week that Russia won’t engage in such “cynical give-and-take.” But he said ratification of the arms pact would “give a powerful impulse” to bilateral ties.