Washington ? President Barack Obama and Russian President Dmitry Medvedev talked Saturday about ongoing negotiations for a successor to an expired nuclear weapons treaty. The Kremlin said the two sides are making “substantial progress.”

An Obama administration official confirmed the two presidents talked by telephone about the negotiations, but had no further details. The official spoke on condition of anonymity because the administration had not announced the discussion publicly.

The Strategic Arms Reduction Treaty, known as START, expired Dec. 5. Both governments have spent the past several months negotiating a new pact that would further reduce the size of the nuclear arsenals on both sides.

The Kremlin statement said the two presidents “continued a detailed exchange of opinions about the results achieved and the prospects of completing work on a document that is of vital importance for strategic stability in the world.”