MOSCOW, July 25, 2009 (AFP) - The United States and Russia ended a fourth round of talks on replacing a key Cold War-era nuclear disarmament treaty and agreed to meet again in a month, the Russian foreign ministry said Saturday.

US and Russian negotiators met in Geneva from Wednesday to Friday to discuss a successor agreement to the 1991 Strategic Arms Reduction Reduction Treaty (START), which expires in December, the ministry said in a statement.

"The sides agreed to carry out the next round of negotiations at the end of August-beginning of September in Geneva," it said.

The statement did not give further details about the talks, except to say that negotiators worked from principles laid out by US President Barack Obama and his Russian counterpart Dmitry Medvedev at a summit earlier this month.

At the summit, Obama and Medvedev agreed on a roadmap for a successor treaty that set numerical targets for further reductions of the nuclear warheads and ballistic missiles in both countries' arsenals.

START, seen as a cornerstone of strategic arms control, expires December 5 and Moscow and Washington have been seeking to thrash out a successor agreement by the end of the year.

But the talks face a number of obstacles, including a bitter dispute over a planned US missile shield in eastern Europe that Russia vehemently opposes.