The warnings over the offensive come as a U.S.-brokered peace process has limped along with inconclusive meetings and little concrete results.

The advance may be linked to Moscow's attempts to create "buffer zones" along the border, Ukrainian regional authorities say.

By Tom Balmforth and Yuliia Dysa KYIV (Reuters) -Russian forces have captured four villages in Ukraine's northeastern Sumy region, the local governor said, the latest battlefield setback for Kyiv as it seeks to hold territory and avoid handing Moscow the advantage in any peace talks.