Copenhagen ? U.S. President Barack Obama and his Russian counterpart, Dmitry Medvedev, struck an optimistic tone even as they conceded that they were unlikely to sign a deal this year on a successor to an expired nuclear arms control treaty, as they had hoped.

The two leaders met Friday as negotiators are seeking to bridge differences on elusive details of a successor to the Strategic Arms Reduction Treaty.

Obama said Friday that they were “quite close.” He had wanted a new deal in place before the end of the year, but that appeared unlikely.

The holdup has denied the White House a quick boost in its efforts to demonstrate improved relations with Moscow. The Obama administration had identified a successor to the START treaty as among the most achievable areas of cooperation with Russia, as it seeks broader help from Moscow on issues including reining in Iran’s suspected nuclear ambitions.