By Tom Balmforth KYIV (Reuters) – The Kremlin refused to deny on Monday that Russia might close its borders to prevent an exodus of military-age men after President Vladimir Putin declared a partial mobilisation in an effort to regain the upper hand in the war in Ukraine. Kremlin spokesman Dmitry Peskov also confirmed that Russia was in “sporadic” contact with the United States on nuclear issues, a day after Washington warned of “catastrophic consequences” if Moscow used nuclear weapons to protect Ukrainian regions it looks set to annex. Citizens in four regions of Ukraine were voting for a fo…