The two sides hope to reach a deal in early 2010, the U.S. said Wednesday.
The American delegation led by Assistant Secretary of State Rose Gottemoeller has left for the United States and will return to Geneva for more negotiations in January, the U.S. diplomatic mission in Geneva said.
"Our goal remains to conclude a solid treaty for the presidents' signature as soon as possible," it said.
Presidents Barack Obama and Dmitry Medvedev had been hoping to sign a new deal before the end of the year, but conceded last week at climate talks in Copenhagen that their goal was unlikely to be met.
The 1991 Strategic Arms Reduction Treaty expired Dec. 5, and both countries have agreed to continue to honor its main provisions until a successor treaty is in place. The START treaty successor is seen as one of the most achievable areas of cooperation. Washington is also seeking help from Moscow on trickier issues such as the standoff over Iran's uranium enrichment program.
START required each country to cut its nuclear warheads by at least one-fourth, to about 6,000, and to implement procedures for verifying that each side was sticking to the agreement.
At a summit in Moscow last July, Obama and Medvedev agreed to cut the number of nuclear warheads on each side to between 1,500 and 1,675 within seven years.
Russian officials have said previously that Moscow wants to simplify START's sprawling web of control measures, which were seen as crucial for both nations to keep an eye on one another's nuclear stockpiles even as the Cold War was ending.
The Kremlin now sees them as too intrusive and unnecessary.