Ukraine continues major incursion into Russia, with reports of advances amid heavy fighting

LONDON -- Ukrainian forces are continuing to advance deeper into Russia's Kursk region, expanding their area of control on the third day of their major incursion, with the situation worsening for Russian forces, according to a key pro-Kremlin Russian military blogger.

Rybar, a blog closely linked to Russia's defense ministry, reported Thursday that Ukrainian armored units have reached the village of Bolshoe Soldatskoe, roughly 18.5 miles inside Russia's border.

Heavy fighting is now also reported only 9 miles from the town of Lgov, which straddles a crucial highway.

"Despite the attempts of the Russian joint forces group to stop the advance of Ukrainian mobile groups, the scale of the crisis is widening," Rybar wrote on Telegram.

Rybar and other pro-Kremlin military bloggers are contradicting the claims of Russia's defense ministry that the Ukrainian advance has been stopped.

Ukraine's attack appeared to be a large-scale offensive operation, involving at least two Ukrainian brigades, rather than a less significant cross-border raid. As the scale of the attack was becoming clearer on Thursday, it appeared to be one of the most significant military developments in the war in months.

At least 66 people have been injured as a result of shelling in the Kursk region since Tuesday, the Russian Ministry of Health reported Thursday.

The railway stations in three settlements in the Kursk region -- Sudzha, Korenevo and Psel -- are closed amid the invasion, the press service of the Moscow Railway reported.

Russian President Vladimir Putin has called for "courage" in the region.

"This requires you, and the current situation requires a certain amount of courage and concentration on ways to solve these complex, difficult, extraordinary tasks that are now facing all branches and all levels of government," Putin said at a meeting with the acting governor of the region, Alexei Smirnov, on Thursday.

Mykhailo Podolyak, a Ukrainian adviser to the head of President Volodymyr Zelenskyy's office, commented Thursday for the first time on the operation, saying one goal was to alter the Russian perception of the war, a shift that could potentially affect any eventual negotiations.

"This increases the cost of the war for Russia quantitatively. More armored vehicles have been destroyed, the Russian Federation has lost territories, and there have been more casualties. Will this affect how they perceive this war? Undoubtedly," Podolyak said in a live discussion on Ukrainian TV.

Zelenskyy praised the Ukrainian army on Thursday for its ability "to surprise" and achieve results, though made no direct reference to the situation in Kursk.

"Everyone can see that the Ukrainian army knows how to surprise. And knows how to achieve results," Zelenskyy said at an event in Kyiv. "This is demonstrated by the battlefield, where our soldiers not only withstood the overwhelming force of the occupiers, but also are destroying it in the way necessary to protect Ukraine -- our state and independence."

The Ukrainian incursion began on Tuesday when a Ukrainian force numbering in at least the hundreds crossed over the border near the village of Sudzha, with tanks and heavy weapons, according to official and unofficial Russian public sources. Catching Russia off-guard, Ukrainian soldiers quickly seized a handful of villages, advancing up to 6 to 9 miles, according to the pro-Kremlin Russian military bloggers.

Since then, Ukraine has moved in significant reinforcements and its forces were continuing to try to press forward but were being held on Thursday at the village of Korenovo, according to multiple pro-Kremlin bloggers, who are close to Russia's military. Heavy fighting was also focused on Sudzha, which Ukrainian troops were reported to have largely surrounded.

Russia's defense ministry on Thursday claimed to have halted the Ukrainian advance and to have inflicted hundreds of casualties on Ukrainian troops. But reports from the Russian military bloggers suggested a far more chaotic situation, with unconfirmed reports that Ukrainian forces had continued to reach deeper in some places into the Kursk region.

One of the best-known pro-Kremlin military bloggers, Two Majors, reported that six or seven Ukrainian tanks were fighting in the village of Ivnitsa, roughly 18.5 miles from the border.

He also reported gunfire, likely from Ukrainian reconnaissance special forces units, in the village of Anastasevka, more than 18.5 miles from the border and about 28 miles from the Kursk nuclear power station.

Multiple military bloggers also reported gunfire, likely from Ukrainian reconnaissance special forces units, in the village of Anastasevka.

They also reported Ukraine engineering equipment to try and dig in and hold ground.

Ukrainian officials have been almost entirely silent on the operation, with speculation swirling around its possible goals. Ukraine may be seeking to pull Russian forces from elsewhere in Ukraine, as Ukrainian troops are under intense pressure in the Donbas region near the key city of Pokrovsk, although most analysts believe Russia likely has sufficient forces to continue its operations there unchanged.

Russian analysts have also suggested that Ukraine could be seeking to seize the Kursk Nuclear Power Plant, which is located roughly 50 miles from the border, but most analysts were deeply skeptical that the Ukrainian force is large enough to reach it.

Podolyak's comments on Thursday appeared to perhaps support another theory that Ukraine could be seeking to capture Russian territory with the goal of trading it for Ukrainian-occupied land in potential future peace negotiations. Podolyak said he expects the Kursk attack to impact Russian society, bringing clear signals of the ineffectiveness of Putin's strategy closer to home, at the same time as potentially strengthening Ukraine's position in negotiations.

"Do they respond to anything other than fear?" he told Ukrainian television. "No, we need to finally realize this. Any compromise is perceived by Russia as your weakness and readiness to kneel before them. When can they sit at the negotiating table, and can something be achieved? Only if they understand that the war is not going according to their plan."

Some Ukrainian and independent military analysts have expressed doubts about the wisdom of such a risky operation when Ukraine is suffering from severe manpower shortages in Donbas, where Russia in recent weeks has been making rapid advances towards Pokrovsk, prompting fears Ukrainian lines near there are in danger of cracking. Russian forces overnight reportedly again made advances in that area, capturing another small village, according to Ukrainian military analysts.

ABC News' Natalia Popova contributed to this report.