MINSK - The U.S. and Russian presidents will sign a new deal to cut Cold War arsenals of nuclear weapons by the year end, but may miss an early December deadline by several days, a Kremlin source told Reuters on Friday.

Diplomats from the world's two biggest nuclear powers are preparing a new agreement on cutting atomic weapons before the 1991 Strategic Arms Reduction Treaty (START-1) expires on December 5.

The new accord will be signed in a European country in December, the Kremlin source told Reuters in Minsk, where President Dmitry Medvedev was meeting regional leaders.

We may not be able to do it by December 5, said the Kremlin source, who did not give a reason for the delay.

Presidents Barack Obama and Medvedev are both due to make visits in Europe in the next few weeks and diplomats say the two sides are trying to find a time when the leaders can meet to sign the deal.

Finding a replacement for START-1, which was signed a few months before the Soviet Union broke up, is seen by the Kremlin and the White House as a way to reset relations after the friction and rows of recent years.

This treaty is a great move ahead and will improve relations between the United States and Russia, said Roland Timerbayev, a former Soviet ambassador and nuclear arms negotiator. It was too early to make any conclusions about the significance of missing the December 5 deadline, he added.

Obama and Medvedev, who had promised to find a replacement for the deal by the time START-1 expired, agreed in July to cut the number of deployed nuclear weapons by around a third from current levels to 1,500-1,675 each.

But negotiators in Geneva have been battling a myriad of complex technical questions to thrash out a deal. Russia has pushed for big cuts in the number of operational missiles or bombers -- known by experts as delivery vehicles -- although the negotiators have argued over how to define nuclear weapons.

Diplomats on both sides have hailed the talks as evidence that relations between the Obama administration and Moscow are improving, though some analysts have warned that the negotiations are being rushed to ensure a deal by the year-end.

The currently announced cuts would take the United States and Russia only 25 operationally deployed warheads below a range of 1,700-2,200 which both sides had already committed to reach by 2012 under a 2002 treaty.

After the cuts -- which have to be made within seven years of a new treaty taking force -- the United States and Russia will still have enough firepower to destroy the world several times over.

(Reporting by Denis Dyomkin in Minsk and Conor Sweeney in Moscow; writing by Guy Faulconbridge; editing by David Stamp)