Big breakthroughs between Washington and Moscow will have to wait until after the November US elections, with the two sides meanwhile doing "homework" on issues such as missile defence, a top US official said Friday.

"This is an important 'homework' period in the US-Russia relationship," Acting Undersecretary for Arms Control and International Security Rose Gottemoeller told reporters and students at a Moscow university.

"Now we have to think about the future," said the veteran Russia expert. "How are we going to cooperate on missile defences?"

US plans to deploy a NATO-backed missile defence shield in Europe has strongly bothered Russia, which wants a legally binding agreement saying the system would not be used or aimed against it at any time.

Gottemoeller's remarks came just days after US President Barack Obama was overheard telling his Russian counterpart Dmitry Medvedev that he could be more flexible on the controversial issue once he is re-elected in November.

Obama was immediately assailed by his Republican rival Mitt Romney for striking a secret arrangement with Russia, a charge Gottemoeller firmly denied.

"He was not talking about any secret deals, but stating the political realities of 2012," Gottemoeller told reporters after delivering a lecture to hundreds of students at the prestigious Institute of International Relations.

"In terms of big national policy initiatives, it's just the reality that in an election year it is more difficult to accomplish that," she said, adding that instead "there will be a lot of technical work behind the scenes."

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 it could make its own nuclear capabilities less effective and has sought to build a joint system in which it has an equal say, a proposal that NATO rejects.

Gottemoeller said Russia's concerns "need to be seriously considered", adding that she hoped US experts would be able to assuage Moscow's biggest fears.

"We think that once the experts are able to get together and talk, you will become convinced... that the technical capabilities of the system are simply not those that would undermine Russia's strategic offensive forces," she said.

"We don't face a threat from the Russian Federation. The Russian Federation does not face a threat from the US or the NATO alliance. It's the more difficult and hidden threats, the threat of nuclear terrorism, that we have to figure out," she said.

Moscow has been waging a fierce war of words with Washington ever since president-elect Vladimir Putin last year announced plans to return for a third Kremlin term, something he accomplished with a crushing victory at the polls on March 4.

The former KGB spy has since implicated the US military in the killing of Libyan dictator Moamer Kadhafi and accused the State Department of funding Russian opposition protests, the biggest of the past 20 years.

But Gottemoeller said she saw ties improving based on "solid and pragmatic" work on nuclear missile cutbacks agreed under the 2010 new Start treaty and on the Northern Distribution Network, a route to Afghanistan through Russia and its allies in Central Asia.