By Conor Sweeney

MOSCOW (Reuters) - Russia and the United States may sign an agreement to replace the START nuclear weapons treaty during the Copenhagen climate summit, a Russian source familiar with the summit plans told Reuters on Tuesday.

The presidents of the United States and Russia will go to the Danish capital this week to attend the climate conference, and agreement on cutting their arsenals of nuclear arms would signal that previously tense relations are easing.

Russian President Dmitry Medvedev will be joined by his foreign minister, Sergei Lavrov, as part of the presidential delegation, said the source.

The source said the foreign minister would not be going to Copenhagen unless Russia believed the new treaty could be signed with President Barack Obama there.

Lavrov said on December 9 that the treaty would be signed soon, but declined to elaborate.

The White House declined to comment on Tuesday, while the State Department said progress was being made but that it was too early to say when negotiators would cross the finish line.

"We think we're getting very close to an agreement," said State Department spokesman P.J. Crowley. "Our goal is to get this done by the end of the year and we'll just continue to evaluate this on a day-to-day basis."

Washington and Moscow failed to reach agreement on a successor to the 1991 Strategic Arms Reduction Treaty, the biggest agreed nuclear weapons cut in history, before December 5, when the pact had been due to expire. However, both sides agreed it should remain in force indefinitely pending agreement on a successor.

Negotiators in Geneva failed to meet the deadline because they had only spent a few months in talks, compared with years spent formulating previous treaties, said Fyodor Lyukanov, the editor of Russia in Global Affairs.

"I think it is widely anticipated and expected. They didn't meet the desired deadline due to technical details. But, it was sure they would do it before the end of the year," he said.

"Copenhagen is a good opportunity for both presidents to meet, show they are fighting climate change and sign this document. It creates a good atmosphere and is the first legal document in many years signed by America and Russia demonstrating they can do a deal together," he told Reuters.

The START-1 treaty, signed by then-U.S. President George H.W. Bush and Soviet leader Mikhail Gorbachev, took nearly a decade to achieve. Under the deal, Russia more than halved its nuclear arsenal, the Foreign Ministry has said.

Over the past decade, relations between Moscow and Washington became strained over the Iraq war, NATO's eastward expansion and last year's Georgia war, but Obama pledged to improve ties when he became president.

Last July, Obama and Medvedev outlined a framework for the new treaty, restricting deployed strategic warheads to between 1,500 and 1,675 while limiting the number of delivery platforms to between 500 and 1,100.

The United States and Russia would still have enough firepower to destroy the world several times over.

(Reporting by Conor Sweeney; Editing by Dmitry Solovyov and Sonya Hepinstall)