Russia and the US reached an outline arms agreement in July
The US and Russia say they want a new nuclear arms treaty to enter force as soon as possible, after failing to agree a successor to the Start I pact.
The nations uphold the "spirit" of the 1991 Cold War-era treaty despite its end, the US and Russian presidents said in a joint statement.
Talks on a new accord are expected to continue after the treaty expires. at midnight on Friday.
Russia's foreign ministry said "intense efforts" were ongoing on a new treaty.
US President Barack Obama and his Russian counterpart, Dmitry Medvedev, agreed in July that a new treaty should bring deep cuts in nuclear warheads.
It is confusing that Start and Sort run concurrently
World affairs correspondent
Washington has indicated it would like an interim agreement to come into force until a new treaty is negotiated.
The Start I agreement was signed by Mikhail Gorbachev and George Bush senior in the final days of the Soviet Union. It led to deep cuts in nuclear arsenals on both sides.
The BBC's Tom Esslemont, in Moscow, says that in spite of frenetic diplomatic activity and the "reset" of relations between the two sides it was always going to be difficult for a replacement arms control treaty to come into force before Friday's deadline.
Kremlin sources appear optimistic that something can be agreed while President Obama is in Europe next week to receive his Nobel Peace Prize.
But the details of the new, complex agreement have not been finalised. It will also need to be ratified in both parliaments, and that could take months, our correspondent says.
Under the joint understanding signed in July, deployed nuclear warheads will be cut to below 1,700 on each side within seven years of a new treaty - a huge cut on Soviet-era levels.
In a joint statement, the US and Russian presidents said on Friday: "We express our commitment, as a matter of principle, to continue to work together in the spirit of the Start treaty following its expiration, as well as our firm intention to ensure that a new treaty on strategic arms enter into force at the earliest possible date."