US officials said they were studying the report by Russia's state news agency ITAR-TASS that Moscow would provide Tor M-1 short-range missiles to Tehran in a deal worth more than 700 million dollars (600 million euros).

But the State Department official, who asked not to be named, said, "Any arms sales to Iran, that's a source of concern. And there are certain US laws which govern our potential reaction to that."

The official did not elaborate but Nicholas Burns, undersecretary of state for political affairs currently visiting Russia, made it clear in Moscow that Washington was not happy with the deal.

Burns told Echo Moscow radio that he had raised questions about the reported sale with the Russian foreign ministry, which had promised to respond.