US President Barack Obama on Monday told President Dmitry Medvedev that he had little flexibility to address Russia's objections to a US missile defence shield before his November reelection bid.

Obama was picked up on an open mic privately explaining his position to Medvedev in an exchange heard by some reporters, during their meeting on the sidelines of the nuclear security summit in Seoul.

The US leader told Medvedev, in their last meeting before Vladimir Putin is inaugurated president in May, that on all issues, but particularly missile defence it was important for Russia to give him "space."

Medvedev replied "Yeah, I understand. I understand your message about space," according to a transcript of the exchange carried by ABC News.

"This is my last election. After my election I have more flexibility," Obama said. In return Medvedev promised to "transmit this information to Vladimir."

The exchange appeared to indicate that Obama believes he has little leverage to conclude deeply divisive foreign policy election issues in a campaign year, and also that he is confident he will win reelection.

The White House insisted it was committed to implementing the missile defence shield, despite Russian objections but said the longstanding and difficult issues meant it would take time to conclude a deal.

"Since 2012 is an election year in both countries, with an election and leadership transition in Russia and an election in the United States, it is clearly not a year in which we are going to achieve a breakthrough," said deputy national security advisor Ben Rhodes.

"Therefore, President Obama and President Medvedev agreed that it was best to instruct our technical experts to do the work of better understanding our respective positions, providing space for continued discussions on missile defense cooperation going forward."

The US-backed system has been bitterly opposed by Russia and has remained one of the main stumbling blocks in Moscow's recent relations with Washington.

Washington and NATO argue that the missile shield is meant to protect Western nations against missile attacks from potential future nuclear powers such as North Korea and Iran.

Moscow fears the shield could make its own nuclear capabilities less effective and has sought to build a joint system in which it has an equal say.

NATO has dismissed the idea and sought to assure Russia that its nuclear deterrence would remain unaffected.

The alliance's secretary general Anders Fogh Rasmussen Monday said the bloc intended to announce the deployment of the first "interim" phase of a missile defence shield for Europe at a summit in Chicago.

Republicans pounce on Obama plea to Russia for 'space'
San Diego, California (AFP) March 26, 2012 - Republicans pounced Monday on US President Barack Obama's remark that he could be more flexible with Russia over a controversial missile defense deal if he is re-elected in November.

In remarks overheard on an open microphone, Obama told President Dmitry Medvedev that with a hard-fought presidential vote looming, he has little maneuvering room to address Moscow's objections to the US missile shield.

"This is my last election. After my election I have more flexibility," Obama is heard to tell the Russian leader at a meeting in the sidelines of a nuclear security summit in South Korea.

The president's comments triggered criticism from Republicans at home including White House frontrunner Mitt Romney who called them "an alarming and troubling development."

"President Obama signaled that he's going to cave to Russia on missile defense, but the American people have a right to know where else he plans to be 'flexible' in a second term," said Romney, who is campaigning in California.

"Higher taxes, more spending and increased debt are all on the table as long as Barack Obama is in the White House, despite what he says publicly. President Obama needs to level with the American public about his real agenda," he added.

And he said: "This is no time for our president to be pulling his punches with the American people, and not telling us what he's intending to do with regards to our missile defense system, with regards to our military might and with regards to our commitment to Israel and with regard to our absolute conviction that Iran must have a nuclear weapon."

In Washington, the White House explained it was committed to implementing the missile defense shield despite Russian objections, and that longstanding and difficult issues meant it would take time to conclude a deal.

"Since 2012 is an election year in both countries, with an election and leadership transition in Russia and an election in the United States, it is clearly not a year in which we are going to achieve a breakthrough," said deputy national security advisor Ben Rhodes.

"Therefore, President Obama and President Medvedev agreed that it was best to instruct our technical experts to do the work of better understanding our respective positions, providing space for continued discussions on missile defense cooperation going forward."

Opposition Republicans back home were not buying that explanation, however.

"Pres Obama tells Medvedev he'll be more 'flexible' on missile defense -- that's a real "Etch A Sketch" leader!" wrote John McCain, the Republican party's presidential candidate against Obama during the 2008 White House race.

The prominent US senator was making reference to a huge gaffe last week by a top Romney aide, who said his boss would re-set after the primary elections were over, "almost like an Etch A Sketch."

The remark gave ammunition to Romney's rivals for the party's presidential nomination, who accused the former Massachusetts governor of being a closet moderate and a "flip-flopper" on core issues.

Some Republicans said Monday however that it was the Democratic president whose motives were truly suspect.

"What else is the president promising in whispered tones?" needled Brendan Buck, press secretary to Republican House Speaker John Boehner in a Tweet.