Ukrainian units stormed across the border into Russia's western Kursk region on Tuesday morning in a shock attack and have advanced several kilometres, according to independent analysts.
Ukrainian forces posted a video purporting to show them in control of a town near the border, the first pictorial evidence of their cross-border advances.
The thrust caught the Russian military by surprise after months of gradual advances in eastern Ukraine by Moscow's forces.
Russia has deployed additional troops and equipment, including tanks, rocket launchers and aviation units to stop the advancing troops.
Russia's national anti-terrorism committee said late Friday Ukraine had mounted an "unprecedented attempt to destabilise the situation in a number of regions of our country."
It called Ukraine's incursion a "terrorist attack" and said Kyiv's troops had wounded civilians and destroyed residential buildings.
The committee said it was starting "counter-terror operations in the Belgorod, Bryansk and Kursk regions ... in order to ensure the safety of citizens and suppress the threat of terrorist acts being carried out by the enemy's sabotage groups."
Under Russian law, security forces and the military are given sweeping emergency powers during "counter-terror" operations.
The head of the U.N.'s International Atomic Energy Agency urged both sides to show restraint in view of the proximity of the conflict to one of Russia's largest nuclear power stations with four reactors, 60 km from the border.
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Source: PAP, AFP
