MINSK/WASHINGTON - The United States and Russia are unlikely to finish a pact to cut Cold War arsenals of nuclear weapons by a December 5 deadline but still aim to close the deal by year-end, Russian and U.S. sources said Friday.

A Russian Topol-M intercontinental ballistic missile drives through Red Square during Victory Day parade in Moscow in May 2009. After the cuts -- which have to be made within seven years of a new treaty taking effect -- the United States and Russia will still have enough firepower to destroy the world several times over. (AFP/File/Dmitry Kostyukov) Diplomats from the two biggest nuclear powers have been trying to prepare a new agreement on cutting atomic weapons before the 1991 Strategic Arms Reduction Treaty expires.

The new accord will be signed "in a European country" in December, a 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, when asked about when the presidents would sign the deal. The source did not give a reason for the delay.

A U.S. official said the December 5 deadline was ambitious and outstanding disagreements between the two sides made it less likely that a deal would be reached on time.

"There are still unresolved issues that will require further ... discussion and both sides need to move to make it happen by the fifth in a very dramatic way," he said.

U.S. 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 agree a time when the leaders can meet to sign a deal if it is finished in the coming weeks.

When asked when the signing would take place, Russian Foreign Minister Sergei Lavrov refused to give a date, saying the deal "will be signed in accordance with the orders of the presidents based on the timeframes set by them."

The U.S. official said if the deadline passed, a temporary pact could be used as a bridge.

"Both the negotiating teams recognize that first and foremost they need to keep working to try to finalize a deal that is comprehensive, however, should time run out, efforts shall be made to provide for a bridging agreement," he said.

Obama and Medvedev said in a joint statement on April 1 that they intended to find a replacement for the deal by the time START-1 expired, a step the Kremlin and White House say will "reset" relations after the friction of recent years.

A White House spokesman said Obama's national security adviser Jim Jones discussed the treaty with his Russian counterpart, Sergey Prikhodko, in Washington Wednesday.

"As Presidents Obama and Medvedev reaffirmed in Singapore, both are committed to trying to get a post-START agreement concluded by the end of the year," said Mike Hammer, spokesman for the White House National Security Council.

Obama and Medvedev met in Singapore during the U.S. president's trip to Asia this month.

NUCLEAR ARMS DEAL

Roland Timerbayev, a former Soviet ambassador and nuclear arms negotiator, said it was too early to draw any conclusions about the significance of missing the December 5 deadline.

"This treaty is a great move ahead and will improve relations between the United States and Russia," he said.

Hopes of a deal to replace START-1, which was signed just months before the Soviet Union broke up, rose in September when Obama said the United States would roll back a plan to deploy a European missile shield that Moscow had bitterly opposed.

Russia has so far refused to support U.S. calls for the threat of sanctions against Iran, but diplomats say that cooperation between the two former Cold War foes on Iran is good, setting the tone for a START deal.

Negotiators in Geneva have been battling a variety of complex technical questions to thrash out an agreement.

"The delegations of Russia and the United States are working incessantly but not looking at the time," Russia's Foreign Ministry said in a statement.

"The timeframe for signing new agreement is important but does not define the negotiating process; rather, (it is defined) by the striving of the leaders of Russia and the United States to agree a full, properly working bilateral agreement," it said.

Obama and Medvedev agreed in July to cut the number of deployed nuclear weapons by around a third from current levels to 1,500-1,675 each.

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

(Additional reporting by Conor Sweeney in Moscow; writing by Guy Faulconbridge; editing by Charles Dick and Mohammad Zargham)