* Kyiv says fighting rages in Kharkiv region’s border areas * Source says Russia advanced in push to create buffer zone * President Zelenskiy says Ukraine ready to meet assault * No immediate comment from Russia (Adds context, casualties, Zelenskiy quotes, in paragraphs 3-5, 8, 13-15) By Sergiy Karazy, Tom Balmforth and Yuliia Dysa KYIV, […]