"Russia and the United States have agreed a landmark treaty to reduce their nuclear arsenals, according to a Kremlin official. But another Russian official - and the White House - admitted some issues still needed to be worked out over a successor to the 1991 Strategic Arms Reduction Treaty (Start I). Both sides say the follow-up to the last major Cold War arms reduction pact is likely be signed in Prague next month by President Barack Obama and President Dmitry Medvedev. Speaking about the latest breakthrough, a Kremlin official said: ""All documents for the signing of Start have been agreed."" White House spokesman Robert Gibbs said: ""We are very close to having an agreement on a Start treaty. ""But we won't have one until President Obama and his counterpart Mr Medvedev have a chance to speak. There are still some things that need to be worked out.""