Tensions between Russia and the West are hitting a new peak. And in this face-off, Moscow has an extraordinary piece of leverage: a super-sophisticated, bomber-killing missile that it once threatened to sell to Iran.

Last week, Reuters first reported Russia was preparing an oil-for-goods deal with Iran worth up to $20 billion. An unnamed Iranian official told the news service that the barter would include Russian weapons. And that was before further signs of Russia’s shadow invasion of Ukraine emerged Monday, when crowds spontaneously appeared in three major eastern cities to welcome the troops amassed over the border. The Daily Beast reported that associates of Viktor Yanukovych, the deposed and Kremlin-friendly Ukrainian president, were meeting with pro-Russian activists. One keen-eyed photographer captured a man wearing a Russian Airborne forces tee-shirt at one of the protests.

The trade between Moscow and Tehran would alleviate the economic pressure on Iran that the White House has said helped bring the Islamic Republic to the bargaining table. It may even sink the talks President Obama is hoping will persuade Iran to defang its nuclear program.

If those talks fail, then Russia has the leverage to equip Iran with the missile that could defend its centrifuges and reactors from allied air strikes, the S-300.

“I could see as part of this deal [between Tehran and Moscow] that they would agree to transfer advanced missiles to Iran,” said Mark Dubowitz, the executive director of the hawkish Foundation for the Defense of Democracies and an expert in the Iran sanctions. “If [Russian president Vladimir] Putin became angry enough over the West’s financial punishment of Russia, he could put in play the S-300 deal.”

The S-300 has long been Moscow’s top-of-the-line air defense system. The current model is comparable to U.S. Patriot missile batteries. The S-300 deploys sophisticated radars, launch vehicles and missiles to shoot aircraft and even ballistic missiles out of the sky. Russia has also threatened to sell the system to Syria, whose hapless air force was hacked by Israel in 2007, rendering its anti-aircraft defenses useless when Israel bombed the al-Kibar nuclear facility.

In the second term of the George W. Bush, Russia came close to selling and training Iran’s military on how to use the sophisticated S-300 system. But then in 2010, the Russians pulled back from the sale during negotiations over U.N. Security Council resolution 1929, the resolution the Obama administration used to persuade banks and finance ministries all over the world to isolate most of Iran’s economy.

Moscow ended up supporting that resolution and cancelling the sale—which was considered a triumph of the Obama administration’s foreign policy at the time. But Russia also negotiated an important loophole. While the resolution bans almost every possible arms sale imaginable, it still technically allows U.N. member states to sell Iran air defense weaponry such as the S-300 system.

“There was no prohibition of the S-300 in the resolution,” said Michael McFaul, who left his post earlier this year as the U.S. ambassador to Russia and played a role in 2010 as a senior White House staff member in negotiating the Iran resolution. McFaul said Russia’s president at the time, Dimitry Medvedev, at first privately and then publicly said the spirit of the resolution would prohibit the sale of the S-300. “But he was not obligated to do that by the resolution itself,” McFaul said.

McFaul declined to comment on whether he suspected Russia would actually provide Iran with the air defense system. Dubowitz, however, says he is concerned Moscow could renege on its promise not to sell Iran the S-300.

One Obama administration official told The Daily Beast the United States has seen no evidence to date that Russia would renege on its promise not to sell Iran the S-300 system.

But signals from Moscow and Tehran have already drawn concern from Congress. On Monday, the two senators who drafted the crippling sanctions legislation Obama has implemented against Iran urged the White House to re-impose some of the sanctions it temporarily lifted this fall when the nuclear talks with Iran began.

In the letter, Sen. Mark Kirk, a Republican, and Sen. Robert Menendez, a Democrat, wrote that they were alarmed the barter agreement reported by Reuters “may provide for the transfer to Iran of items of significant value to Iran’s military and its nuclear program.”

For now, the Obama administration is hoping that Russia will consider the potential costs to its own economy if it continues to defy the west in Ukraine. Speaking to reporters on Monday, State Department spokeswoman Jen Psaki pointed to how Russia’s economy has already experienced turbulence after Russia’s annexation of Crimea.

“The World Bank has warned that Russia’s economy could shrink by 1.8 percent this year even without additional economic sanctions,” she said. “The Russian currency has experienced sharp volatility between March 3rd and April 7th. The Central Bank of Russia spent $25.8 billion to prop up the ruble. All of these are specific impacts that we’re seeing in the Russian economy.”

Western sanctions against Russia so far have been limited and focused on banning the travel and seizing the assets of senior officials in President Vladimir Putin’s inner circle. President Obama, however, has also held out the prospect that sanctions could be much tougher against Russia if its military invades eastern Ukraine. Indeed, the executive order Obama signed implementing the first round of sanctions also said the United States was prepared to implement much tougher sanctions on key sectors of Russia’s economy such as mining.

Zachary Goldman, the executive director of the center on law and security at the New York University School of Law, said Russia has considerable leverage on western countries, but not as much as the United States and Europe have on Russia.

“Can the Russians impose pain on the U.S. and its allies in Europe? Probably,” said Goldman, who served as a policy adviser in the Treasury Department’s office that tracks terrorist financing. “Can it impose as much pain on us as we can to them? Probably not.” Goldman however pointed out that Russia could jeopardize western investments in Russia, a knotty international law problem that often takes months if not years to resolve.

The United States also needs Russia in the negotiations with Iran over its nuclear program. Another round of those talks is scheduled to start this week in Vienna, Austria. Russia participates in those talks along with China, France, Germany and the United Kingdom. U.S. officials tell The Daily Beast that to date Russian diplomats participating in the Iran negotiations have not drawn any linkage between them and the stand off over Ukraine. But if the pressure mounts on Moscow, then the West may end up paying the price for punishing Russia, at the bargaining table with Iran.