Ukraine has launched a surprise ground assault into Russia with troops and armored vehicles, Russian officials and independent military analysts said on Wednesday, in what could be one of the largest Ukrainian incursions onto Russian soil in more than two years of war.

The assault, which began on Tuesday into the Kursk region of western Russia, appeared to have resulted in heavy fighting, according to images from the battlefield verified by independent military analysts and Russian statements. Russia’s defense ministry said midday on Wednesday that the fighting was continuing.

Videos verified by The New York Times showed armored vehicles being struck several miles inside Russia, and Moscow said it had rushed troops and fighter jets to respond. Russia’s defense ministry said its forces had “prevented the enemy from advancing deep into the territory of the Russian territory,” while pro-Kremlin military bloggers said Ukrainian forces had captured several settlements near the border.

The various reports could not be independently verified, and the Ukrainian authorities have not commented on the assault. Two spokesmen for the Ukrainian army did not immediately respond to requests for comment.

This is not the first time that Ukraine has staged ground attacks across the border into Russia. But while previous assaults were carried out by armed groups of Russian exiles backed by Kyiv’s army, the attack on Tuesday appeared to have directly involved Ukrainian troops, according to Pasi Paroinen of the Black Bird Group, which analyzes footage from the battlefield.

Mr. Paroinen said that a few hundred Ukrainian troops, supported by armored vehicles, had most likely crossed the border.