Fierce fighting raged inside Russia on Thursday after Ukrainian armored columns reportedly advanced as far as six miles into Russian territory and captured several small settlements.
The incursion, which was filmed on videos posted online and reported by Russian officials and military analysts, prompted the local governor to declare a state of emergency.
On Wednesday President Vladimir V. Putin of Russia convened with top military and security officials to strategize about the attack.
The surprising assault appears to represent an effort by Ukraine to turn the tables on Moscow by pushing into its territory. In May, Russia caught Ukraine off guard when it sent troops across the border in the area north of Kharkiv, where it still has a narrow foothold.
Ukraine has remained mostly silent about the attack, which began on Tuesday.
The Ukrainian military has not acknowledged its troops are in Russia.
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