By Tom Balmforth KYIV (Reuters) -Ukrainian forces achieved their biggest breakthrough in the country’s south since the war began, bursting across Russian lines on Monday and advancing rapidly along the Dnipro River, threatening supply lines for thousands of Russian troops. Kyiv gave little information about the gains, but Russian sources acknowledged that a Ukrainian tank offensive had advanced dozens of kilometres along the river’s west bank, recapturing a number of villages along the way. The breakthrough mirrors recent Ukrainian successes in the east that have turned the tide in the war aga…