Republicans pounced Monday on US President Barack Obama's remark that he could be more flexible with Russia over a controversial missile defense deal if he is re-elected in November.

In remarks overheard on an open microphone, Obama told President Dmitry Medvedev that with a hard-fought presidential vote looming, he has little maneuvering room to address Moscow's objections to the US missile shield.

"This is my last election. After my election I have more flexibility," Obama is heard to tell the Russian leader at a meeting in the sidelines of a nuclear security summit in South Korea.

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 signaled that he's going to cave to Russia on missile defense, but the American people have a right to know where else he plans to be 'flexible' in a second term," said Romney, who is campaigning in California.

"Higher taxes, more spending and increased debt are all on the table as long as Barack Obama is in the White House, despite what he says publicly. President Obama needs to level with the American public about his real agenda," he added.

And he said: "This is no time for our president to be pulling his punches with the American people, and not telling us what he's intending to do with regards to our missile defense system, with regards to our military might and with regards to our commitment to Israel and with regard to our absolute conviction that Iran must have a nuclear weapon."

A Romney campaign spokeswoman later told AFP that Romney misspoke and meant to say Iran should not have a nuclear weapon.

In Washington, the White House explained it was committed to implementing the missile defense shield despite Russian objections, and that longstanding and difficult issues meant it would take time to conclude a deal.

"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," said deputy national security advisor Ben Rhodes.

"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."

Opposition Republicans back home were not buying that explanation, however.

"Pres Obama tells Medvedev he'll be more 'flexible' on missile defense -- that's a real "Etch A Sketch" leader!" wrote John McCain, the Republican party's presidential candidate against Obama during the 2008 White House race.

The prominent US senator was making reference to a huge gaffe last week by a top Romney aide, who said his boss would re-set after the primary elections were over, "almost like an Etch A Sketch."

The remark gave ammunition to Romney's rivals for the party's presidential nomination, who accused the former Massachusetts governor of being a closet moderate and a "flip-flopper" on core issues.

Some Republicans said Monday however that it was the Democratic president whose motives were truly suspect.

"What else is the president promising in whispered tones?" needled Brendan Buck, press secretary to Republican House Speaker John Boehner, in a Tweet.