U.S., Russia focus on details of potential arms pact

* Technical talks centre on fine print of possible pact

* Silence in Geneva, optimism in Moscow about "START 2"

* Obama's visit to Russia could give push to accord

GENEVA, June 3 (Reuters) - U.S. and Russian officials on Wednesday wrapped up three days of technical talks about a possible new pact on cutting stockpiles of ageing nuclear weapons left over from the Cold War.

Although neither side in Geneva would comment on the discussions, the negotiations ended amid cautiously optimistic signs from Moscow that an accord replacing the cornerstone 1991 Strategic Arms Reduction Treaty (START 1) could be ready soon.

"The current three-day round of talks on an agreement to replace the START treaty has concluded in Geneva," said a statement from the U.S. diplomatic mission. "The talks are over, that is all that can be said," said a Russian official.

Russian sources said the five working sessions concentrated on ways to assess stockpiles of the weapons that once threatened to plunge the world into nuclear conflict but were partly mothballed after 1991.

"The full-scale discussions will now go back to the major negotiators in Moscow and Washington," said one source.

Kremlin spokeswoman Natalya Timakova told reporters in Moscow on Tuesday that it was not excluded that at least an outline deal could be wrapped up in time for a visit to Moscow by U.S. President Barack Obama on July 6-8.

U.S. and Russian negotiators, who have been working on a new START accord for over a year with little sign of movement until Obama replaced former president George W. Bush in January, are working against a year-end deadline.

START 1 -- which came as the culmination of the major thaw in U.S.-Soviet relations initiated by Kremlin leader Mikhail Gorbachev and President Ronald Reagan of the United States in the late 1980s -- runs out on December 5.

Although diplomats say it is highly unlikely that either power would abandon its provisions, completion of a START 2 would be seen as heralding a warming of the frosty relations between them during much of the Bush presidency.

In Moscow, Obama is to have his first face-to-face talks with Russian President Dmitry Medvedev. Both have said a new deal should provide for arsenal reductions to below the 1,700 to 2,200 warhead limit set for 2012 under an interim 2002 treaty.

But Russia has warned that the United States must allay its concerns over Washington plans conceived under Bush to station elements of an anti-missile system in Poland and the Czech Republic, once part of the old Soviet empire.

Bush officials said the system would provide a shield against rockets fired by what it saw as rogue states, like Iran, but Russia argues it could be used to underpin nuclear blackmail against Moscow by a more hard-line U.S. administration. (Editing by Richard Balmforth)