U.S. officials said Monday that they want answers from Russia on whether it is selling advanced surface-to-air missiles to Iran, a move the U.S. insists could threaten American troops in Iraq and Afghanistan. A senior military intelligence official said that while Moscow has sent out conflicting responses to reports on the sale of long-range S-300 missiles, the U.S. believes it is taking place. However, it appears that no equipment has yet been delivered to Iran, said the official, who spoke on condition of anonymity because of the sensitivity of the matter.

Russia''s state arms export agency said Monday it is supplying Iran with defensive weapons, including surface-to-air missiles, but did not say whether they include sophisticated long-range S-300 missiles.

State Department spokesman Robert Wood said the U.S. is seeking clarification from Russia.

"We have repeatedly made clear at senior levels of the Russian government that we would strongly oppose the sale of the S-300," Wood said. "As the U.S. government has said before, this is not the time for business as usual with the Iranian government."

Iran currently has an antiquated missile defense system, dating back to the 1960s and 1970s, so the Russian sale would provide Tehran a much longer range, more mobile and lethal capability. With a range of roughly 75 miles, the Russian system would allow Iran to reach coalition forces operating in Iraq and Afghanistan, if the missiles were moved near the borders.

Both the U.S. and Israel have strongly opposed the sale, saying that supplying such an advanced anti-aircraft system to Iran would shift the military balance of power in the Middle East. It also would make any strike at Iran''s first nuclear power plant — which Russia is helping to build — more difficult.