KYIV, Ukraine >> Russia is pushing back against Ukraine’s largest assault into Russian territory since the start of the war, sending troop reinforcements, establishing strict security measures in border areas and conducting airstrikes, according to the Russian Defense Ministry. One of the strikes on Ukrainian troops involved a thermobaric missile that causes a blast wave and suffocates those in its path, the ministry said.

But even as Russia has halted the quick advances made by Ukrainian troops who launched a surprise cross-border attack five days ago into the southwestern region of Kursk, Ukrainian forces seem to be holding ground.

They claimed Saturday to have captured a small village in the neighboring Belgorod region, and analysts say their forces control most of the Kursk town of Sudzha, about 6 miles from the border.

Pasi Paroinen, an analyst from the Black Bird Group, a Finland-based organization that analyzes satellite imagery and social media content from the battlefield, said in an interview that evidence is now suggesting that Moscow had been able to stall the major advances in Russian territory late in the week.

“We’re now entering the phase where the easy gains have been made,” he said of Ukraine’s initial advance.

“This phase, for the first three days, saw the most rapid movement,” he added. “And yesterday, I think, we started to see the effects of the Russian response.”

What all of this means for Ukraine is not yet clear. In the third year of a war that has seemed largely frozen along a 600-mile front line in eastern and southern Ukraine, the decision by Ukrainian forces to cross the border into Russia apparently surprised not just Russia, but also the United States, other Western partners and analysts who spend their days following the war’s troop movements.

Some have speculated that Ukraine hopes to draw Russian troops away from the front lines in Ukraine, giving battle-weary Ukrainian troops a needed rest, although analysts say that has not happened.

But for Russia, the fallout from last week’s incursion has been a public black eye for President Vladimir Putin.

Andrei Gurulyov, a retired military officer who is now a member of parliament, on Friday condemned Russia’s response and level of preparedness.

“There is no military system in place for guarding the state border, no reserves and no second lines of defense,” he said on Telegram, adding, “If the Ukrainian Armed Forces spent two months preparing for this, how did we miss it?”

Dara Massicot, a senior fellow at the Carnegie Endowment for International Peace in Washington, said that the Ukrainian incursion represented for Russia “a system failure on multiple levels.”

“Border regions were placed on an emergency footing in 2022 to be specifically postured for these kinds of contingencies,” Massicot said, “which makes the lapse even more glaring.”

To counter the incursion, Russia’s military appears to be relying mostly on units that were already deployed near the area, according to an analysis by the Institute for the Study of War, a Washington-based think tank. Most of those units consist of military conscripts and irregular forces.