Barack Obama's rivals for the White House have pounced on controversial comments he thought he had made in private to the Russian President Dmitry Medvedev.
The US President was overheard on an open microphone telling Mr Medvedev that he could be more flexible in negotiations on the issue of missile defence if he is re-elected in November.
He said: "This is my last election. After my election I have more flexibility."
The exchange happened on the sidelines of a nuclear security summit in South Korea and Mr Medvedev is heard to respond positively in English.
He said: "I understand. I will transmit this information to Vladimir."
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 signalled that he's going to cave to Russia on missile defence but the American people have a right to know where else he plans to be 'flexible' in a second term," he said.
"President Obama needs to level with the American public about his real agenda."
A spokesman for Mr Obama's re-election campaign spokesman accused Romney of "undermining his credibility by distorting the president's words".
And the White House has been desperately trying to limit the damage of the overheard remarks by insisting it is committed to implementing the missile defence shield despite Russian objections.
Deputy national security advisor Ben Rhodes said: "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."
Mr Obama made light of the incident at the start of the Nuclear Security Summit in South Korea.
He was about to sit down when he saw Mr Medvedev two seats away.
Mr Obama said: "Wait, wait, wait, wait." He then moved to cover up his microphone in jest.
Russia has strongly criticised plans for a US-led Nato missile defence shield in Europe, claiming it would target its nuclear deterrent and undermine global stability.
The US has said the planned missile shield is intended to counter threats from Iran .
Republican congressman Mike Turner, the chairman of the House of Representatives' Armed Services Subcommittee on Strategic Forces, has written to Mr Obama demanding an "urgent explanation" of his comments.
At last year's G20 summit, an open microphone caught Mr Obama and French president Nicolas Sarkozy discussing their relationships with Israeli prime minister Benjamin Netanyahu.
:: Russian bloggers have ridiculed Mr Medvedev, who is widely seen as a lame duck president, over his promise to "transmit" Mr Obama's open-mic message to Vladimir Putin, who reassumes the top job on May 7.
"Vladimiru", the Russian for "to Vladimir", became a worldwide Twitter trend in a just a few hours as it was used as a universal response to any sort of statement or demand.
Typical was "I am Dima, I don't want to make any decisions. I will transmit to Vladimir", a tweet by @sul, using the familiar name for Dmitry.