* Medvedev: Sometimes sanctions are inevitable
* Six powers to meet at UN on Iran's nuclear program
* Sarkozy warns Iran against "tragic mistake"
UNITED NATIONS, Sept 23 (Reuters) - U.S. President Barack Obama said he and Russian President Dmitry Medvedev agreed on Wednesday that serious additional sanctions must be considered if Iran does not respond to proposals to end a nuclear standoff.
Foreign ministers from the five permanent U.N. Security Council members -- the United States, Russia, Britain, France and China -- and a German official were gathering at the United Nations to continue discussions about Iran's nuclear program, which the West suspects is aimed at developing atomic weapons.
Tehran insists its nuclear ambitions are limited to the peaceful generation of electricity and has rejected Security Council demands that it suspend all sensitive nuclear activities, including its uranium enrichment program.
"Iran has been violating too many of its international commitments," Obama said after meeting Medvedev.
Medvedev made clear that Moscow was ready to back further sanctions against the Islamic Republic unless it changes course on its nuclear program, despite Russia's general reluctance to support such punitive measures.
"Russia's position is simple," he said. "Sanctions are seldom productive but they are sometimes inevitable."
Earlier, French President Nicolas Sarkozy told the U.N. General Assembly that Iran would be making a "tragic mistake" if it thought the world would not respond to its nuclear program.
The six powers meeting at the United Nations have offered Iran economic and political incentives if it stops enriching uranium but Tehran has yet to officially reply. In a television interview, Sarkozy said Iran has until the end of the year to respond.
Iranian President Mahmoud Ahmadinejad, whose disputed re-election in June has sparked widespread protests in Iran, is due to address the assembly later. Hundreds of Iranians picketed Iran's U.N. mission in New York, vowing not to accept Ahmadinejad as their leader.
RUSSIAN REVERSAL ON SANCTIONS
Russian Foreign Minister Sergei Lavrov is expected to join his counterparts from the other five countries to discuss their Oct. 1 meeting with an Iranian delegation in Geneva. It is the six powers' first meeting with Iran since July 2008.
Russia had previously ruled out new sanctions against Iran but appears to have reversed its position.
Obama announced last week that he would scrap plans for a U.S. missile shield in the Czech Republic and Poland that had angered Russia. Washington and Moscow have denied any quid pro quo deal on Iran sanctions in exchange for scrapping the shield, but analysts and U.N. diplomats suspect otherwise.
The previous U.S. administration said the shield would protect against an attack by Iran but Moscow suspected it was the target. Moscow's anger about the project was one of the reasons U.S.-Russian relations sank to a post-Cold War low.
Russia and China reluctantly backed three council resolutions imposing sanctions against Iran over its nuclear program but worked hard to dilute the measures.
Iranian Foreign Minister Manouchehr Mottaki met with British Foreign Secretary David Miliband at the United Nations on Tuesday. Mottaki said afterward that the Western powers knew sanctions were a failed policy.
"If they would like to taste once again the failed policies, that is up to them," he said.
The United States and Germany have called for targeting Iran's energy sector if it refuses to suspend its enrichment program. The U.N. sanctions in place against Tehran focus on its nuclear and missile industries.
Israeli Prime Minister Benjamin Netanyahu, speaking on NBC's "Today" show, repeated U.S. Secretary of State Hillary Clinton's call for "crippling sanctions" against Iran.
Asked about a possible Israeli military strike against Iran's nuclear facilities, he gave what has been his stock answer during his U.S. visit: "I don't deal in hypotheticals." (Additional reporting by Louis Charbonneau, Parisa Hafezi, Jeffrey Heller and Andy Quinn, and Crispian Balmer in Paris; writing by Louis Charbonneau, editing by Doina Chiacu)
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