US President Barack Obama Tuesday hit back at Republicans who pounced after he was overheard telling Russia's leader he could be more flexible over a missile defence row following November's election.

Republicans said Obama's comments in Seoul Monday to Russian President Dmitry Medvedev -- picked up by a live microphone -- suggested that he planned to cave in to Moscow if re-elected.

But Obama denied he was "hiding the ball" from US voters who will be asked to decide in November on his request for a second White House term.

And he complained that the aggravated political atmosphere in the United States did not permit the kind of bipartisan discussion needed to pursue delicate agreements on issues like defence and arms control.

Obama said to build trust with Russia that could lead to new reductions in former Cold War nuclear arsenals, he needed to engage Moscow on its contention that the missile shield infringes its strategic interests.

"The only way I get this stuff done is if I'm consulting with the Pentagon, with Congress, if I've got bipartisan support," Obama told reporters, who said his comments to Medvedev were consistent with his public position.

"Frankly, the current environment is not conducive to those kinds of thoughtful consultations. I think the stories you guys have been writing over the last 24 hours (are) pretty good evidence of that."

Earlier, Obama made light of the controversy, and playfully covered up a microphone during an appearance with Medvedev and other world leaders at talks designed to deprive terrorists of the materials for nuclear arms.

On Monday, after a bilateral meeting with Medvedev he had said: "This is my last election. After my election I have more flexibility."

The Russian leader promised to transmit the US president's position to president-elect Vladimir Putin.

Obama's most likely Republican challenger, Mitt Romney, immediately sought to make political capital out of the comments.

"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," said Romney, who was campaigning in California.

The Obama campaign mounted a counter-attack and tried to flip the issue back into a critique of Romney's approach to foreign policy.

"Once again Governor Romney is undermining his credibility by distorting the President's words," said Obama campaign spokesman Ben LaBolt.

"Governor Romney has been all over the map on the key foreign policy challenges facing our nation today, offering a lot of chest thumping and empty rhetoric with no concrete plans to enhance our security or strengthen our alliances."

Obama's push for reelection is complicated by the fact that many Americans have yet to feel the economic rebound after the worst recession since the 1930s Great Depression, despite a drop in unemployment.

But many analysts say he has a good case to make on his record on foreign policy and as commander-in-chief, so Republicans are quick to seize on any perceived foreign policy gaffes.

The White House says it is committed to implementing a missile defence shield in Europe despite Russian objections, but that longstanding hurdles mean it would take time to conclude a deal.

Washington and NATO argue that the shield is meant to protect Western nations against missile attacks from potential future nuclear powers such as Iran. Moscow fears the shield could undermine its own nuclear capabilities.