Since its surprise incursion into Russia more than a week ago, Ukraine has steadily gained ground, saying it advanced even deeper into Russian territory on Wednesday. It says it has captured hundreds of soldiers, as Russia has evacuated more than 130,000 people from nearby communities and declared a new state of emergency in one region.

And now Moscow has begun withdrawing some troops from Ukraine in an effort to repel Kyiv’s offensive into western Russia, according to U.S. and Ukrainian officials, an indication that the surprise attack is forcing Moscow to change its battle plans in Ukraine.

Taken collectively, Ukraine has deftly put Russia on the defensive, creating a new, if small, frontline in a war where Moscow has long had the upper hand. If Russia brings reinforcements in large numbers from other parts of the front, it could provide some relief to Ukrainian troops who are struggling to push back relentless Russian attacks, particularly in eastern Ukraine.

It is an edge that Ukraine appears intent on keeping, as its eyes, for now, holding the Russian territory it controls. President Volodymyr Zelensky discussed on Wednesday the possibility of establishing military administrations in areas seized by Ukrainian troops. The country’s interior minister, Ihor Klymenko, said Ukraine was creating a “buffer zone” in western Russia to protect border towns and villages.But it’s an advantage that may not translate into broader gains elsewhere — and one that may be hard to maintain.

Military analysts say Russia has so far responded by sending units that were not fighting in hot spots on the front, making it unclear whether Ukraine’s gambit would have the effect it desired on the overall battlefield. Russia has been careful not to pull troops out of eastern Ukraine, where its army has been steadily advancing in recent months.

As the incursion onto Russian soil enters its second week, the Ukrainian military will also face its own challenges, the analysts say. Capturing more land will become harder as Russian reinforcements arrive, while holding on to captured territory will expose stationary Ukrainian positions to potentially devastating airstrikes.

The Russian troop movements were reported by Dmytro Lykhovii, a spokesman for the Ukrainian Army, who told the Ukrainian news media on Tuesday that Russia had moved some units from the southern Ukrainian regions of Zaporizhzhia and Dnipro to other areas of the fighting, including the Kursk region.

A U.S. official, speaking on the condition of anonymity to discuss sensitive security matters, also said that Russia had pulled out small numbers of troops from Ukraine, but he did not specify their location or the number of soldiers involved.There is little sign that Moscow has redeployed troops from the Donetsk region of eastern Ukraine, which Moscow hopes to fully capture and where it has been on the offensive for months. Instead, the Russian Army appears to have sent in reinforcements drawn mainly from less combat-ready units based in northern Russia and Ukraine, military experts say.