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

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 pounced Obama's remark that he could be more flexible with Russia over the defense deal if he is re-elected in November.

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.