U.S. President Barack Obama set out Tuesday to clarify private remarks about missile defense to Russian President Dmitry Medvedev inadvertently picked up by a live microphone.

At the Seoul nuclear summit, Mr. Obama privately told the Russian president that this year is his last election and that he would have more flexibility after he is elected.

Mr. Obama later told reporters that the United States is in the middle of a presidential election campaign, and Russia just held a vote and is transferring power.

He said the current volatile political environment is not conducive to sensitive negotiations on missile defense and arms control.

Republicans, who seek to win back the White House from Mr. Obama, a Democrat, in November, have said they do not want to make what they see as concessions to Russia in arms talks.

Russia opposes Mr. Obama's plans for a missile defense system in Europe — an idea first proposed by former president George W. Bush and modified by Mr. Obama to address Russian concerns.

Mr. Obama said Monday that both the United States and Russia can continue to make progress in cutting stockpiles of nuclear weapons.

He called the START agreement ratified in 2010 the most comprehensive arms treaty in 20 years. Mr. Obama said he plans to pursue another round of cuts with Russian President-elect Vladimir Putin.

In another development, Mr. Medvedev criticized U.S. Republican presidential candidate Mitt Romney, who called Russia the “number one geopolitical foe” of the United States.

CNN says Romney commented in response to Mr. Obama's private remarks on missile defense to Mr. Medvedev that were captured by an open microphone.

Mr. Medvedev said Romney's comments reminded him of Hollywood's portrayal of Russia as a U.S. foe. He suggested U.S. presidential contenders should “use their head” when phrasing their positions and remember that it is now 2012 — not the mid-1970s.

Also in Seoul, Mr. Obama, Mr. Medvedev and Kazakhstan's president, Nursultan Nazarbayev, hailed the nearly completed clean-up of an old Soviet nuclear test site.

The three presidents issued a joint statement calling the clean-up efforts a successful example of “trilateral cooperation.” They said it also shows a “commitment to nuclear security and non-proliferation.”

Kazakh, Russian and U.S. experts worked together to clean up and seal off the Semipalatinsk site in Kazakhstan, where the Soviet Union carried out hundreds of nuclear weapons tests during the Cold War.