STORY: Ukrainian forces were charging through an expanding area of previously Russian-held territory in the east on Friday (September 9) after bursting through the frontline in a surprise breakthrough that could mark a major turning point in the war.

After keeping silent for a day, Moscow effectively acknowledged that a section of its frontline had crumbled southeast of Ukraine's second largest city Kharkiv.

Ganchev added a caveat to his statement: "This is what they (Ukrainian forces) wanted to achieve. But once again I say: the question is - at what cost? Very serious forces have been deployed there from Ukraine."

The Kremlin declined to comment on the Ukrainian advance. Such rapid advances have largely been unheard of since Russia abandoned its assault on Kyiv in March, shifting the war mainly into a relentless grind along entrenched front lines.