Obama and Russia's President Dmitry Medvedev have vowed to work towards a successor to the 1991 Strategic Arms Reduction Treaty (START) before it expires on December 5.

Russian Foreign Minister Sergei Lavrov said the presidents would instruct negotiators to keep working toward that goal, but he stopped short of saying that the summit would lead to the concrete announcement of a deal.

"We expect that the presidents will note the progress that has been made and will confirm their instructions to finish this work by December, when the current treaty expires," Lavrov told reporters.

"I am certain that this treaty, in its final version, will establish all the parameters, including numerical ones," he said, answering a question about what numerical ceilings would be imposed on warheads and missiles in a new treaty.

"The talks are proceeding in a constructive and results-oriented fashion," Lavrov said, speaking at a joint press conference with Turkish Foreign Minister Ahmet Davutoglu.

Obama is to visit Moscow from July 6 to 8 and START is expected to be a central topic of his discussions with Medvedev.

Signed in 1991, START led to deep reductions in the US and Russian nuclear arsenals. It limits the number of deployed warheads on both sides to 6,000 and the number of warhead delivery vehicles to 1,600.

Negotiations on a treaty have been partly overshadowed by a dispute over US plans to install missile defence facilities in eastern Europe, which Russia strongly opposes.