MOSCOW - The presidents of Russia and the United States on Friday pledged to keep working for a deal to reduce arsenals of Cold War nuclear weapons, as an existing treaty expired, but did not announce any new agreement.

The White House and the Kremlin say that a replacement for the 1991 Strategic Arms Reduction Treaty (START-1) would help reset relations after the rows of recent years.

Presidents Barack Obama and Dmitry Medvedev said in a joint statement released by the Kremlin that they would continue to work toward a new treaty.

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, the statement said.

Obama and Medvedev agreed in April to work for a deal before the START deal expired at midnight on December 4.

Now diplomats are talking about finding a deal by the year-end, though it is still unclear when the two presidents could meet for a signing ceremony.

Negotiations have been proceeding in Switzerland between the two sides under unusually tight secrecy. Both parties have committed to a news blackout on details of the talks and even senior embassy officials are not being fully briefed.

(Reporting by Guy Faulconbridge, editing by Michael Stott)