Seoul:  President Barack Obama told Russia's leader on Monday that he would have more flexibility after the November election to deal with the contentious issue of missile defense, a candid assessment of political reality that was picked up by a microphone without either leader apparently knowing.

Outgoing Russian President Dmitry Medvedev said he would pass on Obama's message to his successor, Vladimir Putin, according to an audio recording of comments the two leaders made during a meeting in Seoul, South Korea. Obama and Medvedev did not intend for their comments to be made public.

Once they were, the White House said Obama's words reflected the reality that domestic political concerns in the both the US and Russia this year would make it difficult to fully address their long-standing differences over the contentious issue of missile defense.

Obama, should he win re-election, would not have to face voters again.

"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," White House deputy national security adviser Ben Rhodes said.

Tensions over missile defense have threatened to upend the overall thawing of relations between the US and Russia in recent years.

Both leaders acknowledged as much in their public statements to reporters following their meeting. Obama said there was "more work to do" to bridge their differences; Medvedev said each country had their own positions on missile defense, but there was still time to find a solution.

Obama's remarks had immediate repercussions back home. Rep Mike Turner of Ohio, Republican chairman of the House Armed Services Subcommittee on Strategic

Forces, wrote to the President requesting an "urgent explanation of (his) comments to President Medvedev in Seoul this morning."

"Congress has made exquisitely clear to your administration and to other nations that it will block all attempts to weaken US missile defenses," Turner said.

"As the chairman of the Strategic Forces Subcommittee, which authorizes US missile defense and nuclear weapons policy, I want to make perfectly clear that my colleagues and I will not allow any attempts to trade missile defense of the United States to Russia or any other country."

Congress included in the fiscal 2012 defense authorisation act language constraining Obama's ability to share classified US missile defense information with Russia. Obama signed that legislation into law.