Iran has become a key target of U.S. and international sanctions aimed at halting the development of its nuclear program. Still, repeated efforts have been unable to stop Tehran's nuclear ambitions, showing the limitations of sanctions as a foreign policy tool. Experts also point to sanctions against Burma and Cuba as other examples where the measures have failed to achieve their goal. VOA's Brian Wagner has this report in our series on sanctions.

A general view shows the reactor building of the Bushehr nuclear power plant, in southern Iran (File)

The United Nations Security Council voted this month to impose new sanctions on Iran for its refusal to halt uranium enrichment. The agreement is the third set of U.N. measures backed by the United States and Europe to restrict Iran's finances and discourage its nuclear efforts.

Still, Iran's government remains defiant, saying it will continue to pursue its right to develop a nuclear energy program.

Michael Jacobson, is a former official in the Treasury Department's Office of Terrorism and Financial Intelligence. He says U.S. and international sanctions have curtailed Iran's foreign business, but it has not hurt the government.

"There it has been less effective to this point," said Jacobson. "Iran has not completely backed down from its nuclear ambitions and that is the key question is whether or not that will happen."

One reason is that the soaring price of oil has bolstered Iran's government, in spite of a ban on U.S. oil firms investing in Iran. Gary Hufbauer of the Peterson Institute for International Economics says oil revenues have enabled Iran to purchase technology and hire nuclear engineers, often from the former Soviet Union.

"So they have a lot of money and any sanctions we do is marginal to that amount of money," said Hufbauer.

Despite the latest U.N. agreement on sanctions, Western nations have been pushing for even stronger measures at the world body. But Iran's trading partners, especially Russia and China, have voted to reject some of the toughest U.N. proposals.

Hufbauer says the impasse between Western allies and Asian partners has allowed Iran to further its nuclear efforts.

"Iran is getting quiet but definite support from Russia and China -- major powers, and when that is at play, it is pretty definitive," he said.

Armed Burmese security forces march down streets of downtown Rangoon, 27 Sep 2007

Sanctions against Burma have also had limited success at forcing the military government to halt human rights violations and repression of its people. A sharp crackdown on protests led by Buddhist monks and pro-democracy activists last September demonstrated firm control, in spite of sanctions.

Aung Din, policy director for the U.S. Campaign for Burma in Washington, says sanctions are important, but must be combined with other tools.

"Sanctions alone cannot change our country," he said. "Sanctions should combine with a strong diplomatic effort and the strong [democracy] movement inside the country."

New laws pending in the U.S. Congress would tighten the economic restrictions and create new support for democratic initiatives in Burma. Aung also wants to see Western nations actively engage Burma's government and its allies, especially China and India, to try to broker a deal with Burma's opposition groups.

No U.S. sanctions have endured as long as the economic and trade embargo on Cuba. After 46 years of sanctions, the island's communist rulers remain in power, and the U.S. policy has become a frequent target of criticism.

In Miami, some Cuban exiles have begun to challenge the embargo, saying it only hurts Cubans on the island. They chafe at a 2004 law that limits Cubans residing here from sending remittances and making visits to family on the island.

Alvaro Fernandez, head of the Cuban-American Commission for Family Rights, says the rules are splitting apart families.

"Their argument is that it helps the Cuban government," said Fernandez. "More important to me, I want to help my family."

Raul Castro flashes the victory sign during a session of Cuba's National Assembly in Havana, 24 Feb 2008

With the recent transfer of power from Fidel Castro to his brother, Raul, some have questioned the embargo's future. U.S. officials say lifting the measures now would appear to reward Cuba's government for maintaining its anti-democratic policies.

While imperfect, some say the embargo remains one of few tools Washington can use to pressure Cuba's government to change.

"I don't think from an academic point of view that I would be ready to give up my last piece of paper, as weak as it is, without asking for something in return," Andy Gomez, a senior fellow at the University of Miami.

U.S. officials say they recognize the need to make sanctions more effective, but they are unlikely to abandon them as a foreign policy tool any time soon.