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.
Many Republicans have strenuously opposed the deal's weakening of sanctions -- in exchange for certain limits on Iran's nuclear activity -- and have highlighted its lifting of a U.N. ban on arms sales to Iran in the future.
However, under the current prohibition, it is offensive weapons that are barred by the United Nations, Kirby noted. The S-300 system is defensive in nature and allowed.
Kirby, however, indicated that unilateral American sanctions could still be an issue if the sale goes through.
"We need to know more about the specifics of this proposed transfer to determine whether any domestic U.S. sanction programs may be implicated," he said.
Secretary of State John Kerry is raising U.S. concerns with the Russians directly after Iranian Revolutionary Guard Commander Qasem Soleimani recently traveled to Moscow.
The United States doesn't know for certain if he was there to finalize the sale.
If it does take place, it could put the entire region even more on edge.
"The airspace will be more tense," said Michael O'Hanlon, a defense policy expert with the Brookings Institution, of an Iran outfitted with the Russian missile defense system. "It will be filled up with more radar beams if you will -- invisible, and yet ever present, and they will be ... radar signals that we can't easily stop or evade."
The potential sale is hardly the only point of friction between the United States and Russia. The roiling conflict in Ukraine has also raised American hackles.
The United States and NATO have 5,000 troops conducting the largest airborne training exercise in Europe since the Cold War, making sure they can jump into a battlefield under fire and fight together.
"Along with economic tools and diplomatic tools, I think Russia certainly has (received) the message" from the West about its feelings concerning Moscow's moves, said Lt. Gen. Mark Hertling, a national security analyst for CNN.
But with escalating Russian-backed violence in eastern Ukraine, Russian President Vladimir Putin may not be listening.
After a meeting Wednesday to discuss the sharp escalation in the pro-Russian separatist violence, NATO officials issued a statement saying that, "Russia has a special responsibility to find a political solution. Any attempt by the Russian-backed separatists to take over more of Ukraine's territory would be unacceptable to the international community."