U.S. plans to deploy a missile defense system in Eastern Europe continue to be a major source of contention between the United States and Russia.

U.S. officials say the proposed missile defense system - made up of 10 missile interceptors in Poland and a radar station in the Czech Republic - is needed to counter potential threats from countries such as Iran. They say it is not targeted against Russia. But senior Russian officials have strongly criticized the proposed missile defense system.

The United States and the European Union believe Iran's uranium enrichment program is designed ultimately to build nuclear arms. Tehran says it only wants to use its enriched uranium for peaceful purposes, such as generating electricity.

Experts say Russia and Iran have strong commercial ties. Moscow is one of Tehran's largest trading partners, providing it with weapons as well as consumer goods. Moscow is also building a nuclear power plant at Bushehr, in southwestern Iran. And on the political front, Moscow has been a strong supporter of Iran, opposing any tough sanctions against Tehran over its alleged nuclear weapons program.

Recent Russian and American news reports have indicated President Barack Obama - in a letter to Russian president Dmitri Medvedev - offered to back away from deploying the missile defense shield in exchange for Moscow's help in preventing Iran from developing long-range weapons.

Both presidents denied there was any deal. But in an exchange with reporters Tuesday (March 3) Mr. Obama also said:

"To the extent that we are lessening Iran's commitment to nuclear weapons, then that reduces the pressure for or the need for a missile defense system," said the president.

Reva Bhalla with STRATFOR, an independent research organization, said Mr. Obama's letter dealing with ballistic missile defense - or BMD - in Europe was leaked at a critical time.

"Essentially, the United States is telling Russia - okay, we hear your concerns on BMD, now we expect you to use your relationship with Iran to place curbs on the Iranian nuclear program, thereby basically undercutting the entire U.S. rationale for BMD in Europe," she said.

Bhalla says the Polish and Czech governments should be worried.

"They are in a very precarious situation right now and they don't like how the United States is kind of wavering on the issue - not only the Poles, but also the Baltic states who are demanding the United States take a stronger stance," Bhalla said.

On the issue of Iran, experts say whether Tehran decides to curtail its nuclear weapons ambitions could be based on how much pressure Russia exerts on Tehran.

Many experts, including Robert Legvold from Columbia University, say Russia's influence over Iran is limited.

"If the route chosen by the United States and Europe is to try to increase the pressure of sanctions, particularly in terms of petroleum and the export of refined petroleum products to Iran, then Russia would become crucial in that respect. But it's not clear that even tougher sanctions with everybody participating will force Iran's hand. So I think at the end of the day, there's a limit to how much the Russians can do in shaping Iran's choice," he said.

But Reva Bhalla from STRATFOR says the Iranians are in a very perilous situation right now.

"The Iranians should be feeling very nervous right now. There is really no love lost between Russia and Iran. Iran remembers really well the brief Soviet occupation of northern Iran during World War Two and they know that Russia's interest in Iran is really borne out of a more technical desire to demand U.S. attention on issues of strategic interest to Russians such as BMD or NATO expansion," she said.

Experts say in the weeks and months ahead, it will be interesting to see whether the new Obama administration will be able to change Iran's nuclear weapons ambitions - with or without Russia's help.