Ukraine has chalked up a string of victories more than a week since blindsiding Russia with a lightning cross-border assault, but the risks are piling up as its troops make plans to hold territory and Russia recovers its footing.
Ukraine poured thousands of troops into the western Russian region of Kursk last week, pulling down Russian flags in towns seized by its soldiers and wresting the war initiative from Moscow for the first time in months.
On Wednesday, officials in Kyiv said Ukraine would use seized Russian territory as a "buffer zone" to shield its north from Russian strikes. Oleksandr Syrskyi, head of the Ukrainian armed forces, said on Thursday that Kyiv had set up a military commandant's office in the occupied part of Kursk, suggesting ambitions to dig in.
The occupied area exceeds 1,150 sq km, Syrskyi said.
Ukraine's goals in Kursk include distracting Russian forces from the eastern Ukrainian region of Donbas, where Russia has made steady advances for months and which it is seeking to take in its entirety, former Ukrainian defence minister Andriy Zagorodnyuk said in an interview.
There is, however, no sign of that happening for now.
Apart from a reputational blow to President Vladimir Putin, the biggest invasion of Russia since World War Two has destroyed Russian forces, captured soldiers who can be traded and created a sore on Russia's flank, said Polish military analyst Konrad Muzyka.
The Russian defence ministry did not immediately respond to a request for comment. Ukraine's defence ministry referred questions to the armed forces, which did not immediately respond.
Russian officials have said the Ukrainian attack on Russian territory is a "terrorist invasion" and that civilian infrastructure was targeted, which Ukraine denies.
Putin said that Russia will deliver a "worthy response" to the attack but that the immediate task is to eject all Ukrainian troops from Russian territory.
Ukraine, which has not said how long it might remain, was "not interested" in permanently taking Russian land, a foreign ministry spokesperson said this week. Putin has said Ukraine wants the territory as a bargaining chip in eventual peace talks.
Serhiy Zgurets, a Kyiv-based military analyst, predicted Ukraine would seek to retain control of the land between the towns of Rylsk, Korenevoye and Sudzha and the border, giving it control of a roughly 20-km-wide strip of Russian territory.
The area, he said, could be defended by a small force using long-range artillery systems and air defences.
"This line is not difficult to defend, given there are few roads and a large number of rivers," Zgurets said, adding that the area could be easily supplied from the Ukrainian region of Sumy across the border.
He said he didn't expect troops to press towards the Russian regional capital of Kursk, something that could expose them to attacks from the flanks.
Muzyka was more circumspect, warning that trying to hold a swathe of Russian land could open up Ukrainian forces to potentially heavy losses, pointing to manpower problems that have dogged Ukraine for months in its war with a much larger foe.
The counter-invasion was "a massive gamble" that in the short term was paying off, Muzyka said.
"But there may soon come a time when costs associated with the attack in the Kursk region will outweigh the benefits, especially given the steady pace of Russian advances in the Donetsk region," he said.
RUSSIA RESPONSE
After a shambolic response to the early days of Ukraine's assault that saw tanks and Western-supplied armoured vehicles among convoys that carried troops across the border, Russia finally appears to have slowed the advances. Ukrainian President Volodymyr Zelenskiy said forces had advanced a few kilometres on Wednesday.
A senior Russian commander said on Thursday that Ukrainian forces had been pushed out of one village in Russia's border region but that Kyiv's forces were still probing along the front.
Satellite images from Planet Lab and Maxar showed multiple, apparently new, Russian-built trenches further from the border in Kursk region.
Russia has so far relied on military units from near Kursk to try to push back the Ukrainian forces, said Pasi Paroinen, an analyst with Finland's Black Bird Group, which studies publicly available footage from the Russia-Ukraine war.
By bringing the war to Russia, Zelenskiy faces the risk of weakening Kyiv's defences along the front in Ukraine while Russia has already sent in thousands of reserves in a bid to expel the Ukrainian soldiers.
Moscow should have enough reserves, Paroinen said, to respond without pulling troops from the most active frontline of the war in the Donbas region.
Russia has been advancing slowly there for months, deploying gliding bombs in huge numbers as well as assault groups that take heavy losses but make small steps forward, former defence minister Zagorodnyuk said.
Far from a letup in fighting in the east, Ukraine on Thursday reported the heaviest fighting in weeks near Pokrovsk and said there was no sign Russian military pressure was receding along the eastern front inside its borders.
In a tacit acknowledgment of mounting pressure, Zelenskiy ordered his top commander on Wednesday to send more weapons to Pokrovsk and Toretsk, another embattled town that Russia is trying to capture.
Ukrainian soldier Dmytro, 36, who was deployed to the Ukrainian side of the Sudzha border during the incursion, said he wanted the war to end quickly, and he hoped the attack on Russia would put Ukraine on a more equal footing in any negotiations.
He said he viewed the incursion as a necessary step to ward off a Russian attack on Ukrainian territory across the border from the Kursk region, but that he also felt uncomfortable with invading foreign territory.
"Honestly, it does not feel great to do what they (the Russians) did," he said.
