The Start treaty led to huge reductions in the Russian and US nuclear arsenals
The US and Russia "are making very good progress" on a nuclear arms reduction pact, a senior US official has said.
Under-Secretary of State William Burns was speaking as Secretary of State Hillary Clinton was due to hold talks on the issue in Moscow.
"We are getting closer," Mr Burns said, but added that he could not tell when an agreement might be reached.
The two nations are trying to replace the 1991 Strategic Arms Reduction Treaty, which expired last December.
The US says it has more than 2,000 nuclear weapons, while Russia is believed to have nearly 3,000.
"I can't predict to you exactly when the agreement will be completed, but we are getting closer," Mr Burns told reporters.
US plans for a missile defence system in Eastern Europe have irked Russia
Mr Burns declined to identify the specific sticking points that emerged during months of talks in Geneva.
He was speaking ahead of talks in the Russian capital on Thursday between Mrs Clinton and her Russian counterpart Sergei Lavrov, and with President Dmitry Medvedev on Friday.
As well as arms control, the US and Russian foreign ministers are expected to discuss the situation in the Middle East and Iran.
Last week, President Medvedev and his US counterpart Barack Obama had a telephone conversation about the nuclear disarmament issue.
The two leaders "expressed satisfaction with the high level of consensus on the basic lines" of the treaty negotiations, the Kremlin said in a statement.
The pair "stressed that it is already possible to set firm dates to submit the draft agreement to the heads of state for their signatures," the statement added, without setting the timeframe.
The two presidents launched talks to replace the Strategic Arms Reduction Treaty (Start) last April with an original deadline of December.
It was part of a Washington-inspired attempt to "reset" relations with Moscow after years of frayed ties under the administration of former US President George W Bush.
The landmark Start pact was signed in 1991 and led to huge reductions in the two countries' nuclear arsenals.
Both sides have agreed to cut the number of warheads they hold to between 1,500 and 1,675 each.
But there have been disagreements on verification measures, how to count weapons and launch systems.
Another bone of contention is the US plan for a missile defence system in Eastern Europe.
President Obama has said his goal is to have a nuclear-free world. He has promised to cut the number and role of nuclear weapons in US security strategy.
The US is hosting a nuclear non-proliferation summit in Washington in April.
All numbers are estimates because exact numbers are top secret.
Strategic nuclear warheads are designed to target cities, missile locations and military headquarters as part of a strategic plan.
Israeli authorities have never confirmed or denied the country has nuclear weapons.
The highly secretive state claims it has nuclear weapons, but there is no information in the public domain that proves this.
The International Atomic Energy Agency reported in 2003 there had been covert nuclear activity to make fissile material and continues to monitor Tehran's nuclear programme.
US officials have claimed it is covertly seeking nuclear weapons.