SEOUL - President Barack Obama was caught on camera on Monday assuring outgoing Russian President Dmitry Medvedev that he will have "more flexibility" to deal with contentious issues like missile defense after the US presidential election.

Obama, during talks in Seoul, urged Moscow to give him "space" until after the November ballot, and Medvedev said he would relay the message to incoming Russian president Vladimir Putin.

The unusually frank exchange came as Obama and Medvedev huddled together on the eve of a global nuclear security summit in the South Korean capital, unaware their words were being picked up by microphones as reporters were led into the room.

US plans for an anti-missile shield have bedeviled relations between Washington and Moscow despite Obama's "reset" in ties between the former Cold War foes. Obama's Republican opponents have accused him of being too open to concessions to Russia on the issue.

Republican presidential candidate Mitt Romney seized on Obama's comment, calling it "alarming and troubling."

"This is no time for our president to be pulling his punches with the American people," Romney said in a campaign speech in San Diego.

As he was leaning toward Medvedev in Seoul, Obama was overheard asking for time - "particularly with missile defense" - until he is in a better position politically to resolve such issues.

"I understand your message about space," replied Medvedev, who will hand over the presidency to Putin in May.

"This is my last election ... After my election I have more flexibility," Obama said, expressing confidence that he would win a second term.

"I will transmit this information to Vladimir," said Medvedev, Putin's protege and long considered number two in Moscow's power structure.

The exchange, parts of it inaudible, was monitored by a White House pool of television journalists as well as Russian reporters listening live from their press center.

The United States and NATO have offered Russia a role in the project to create an anti-ballistic shield which includes participation by Romania, Poland, Turkey and Spain. ? ?

But Moscow says it fears the system could weaken Russia by gaining the capability to shoot down the nuclear missiles it relies on as a deterrent. ?

It wants a legally binding pledge from the United States that Russia's nuclear forces would not be targeted by the system and joint control of how it is used. ?

The White House, initially caught off-guard by questions about the leaders' exchange, later released a statement recommitting to implementing missile defense "which we've repeatedly said is not aimed at Russia" but also acknowledging election-year obstacles on the issue.

"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.

"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," he said.