Story highlights The advanced S-300 air defense system would mean that U.S. or Israeli warplanes likely couldn't sneak into Iranian airspace

Potential sale is not only point of friction between U.S. and Russia; roiling conflict in Ukraine has also raised American hackles

Washington (CNN) U.S. officials are concerned that Russia is moving ahead with plans to sell Iran a sophisticated missile defense system that could undercut Washington's ability to challenge Tehran's airspace.

The advanced S-300 air defense system would mean that U.S. or Israeli warplanes likely couldn't sneak into Iranian airspace if they wanted to bomb Iran's nuclear facilities. Bombing the S-300 radar and missiles first would give the Iranians a warning that an attack would be on the way.

"We've been making very clearly our objections to any sale of this missile system to Iran, as I said, for quite some time, and we'll continue to monitor it closely," State Department spokesman John Kirby said on Tuesday.

The signs that Moscow could soon be completing the sale comes as the United States, Russia, Iran and four other world powers recently completed a deal curbing Tehran's nuclear program.

The agreement is under review by Congress, where Republicans are planning to vote against the deal amid concerns Iran would still be able to obtain a weapon with the agreement in place. The White House has promised to veto such a resolution of disapproval.

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