Ukraine says its hard-pressed military is battling 50,000 troops inside Russia's Kursk region while also scrambling to reinforce two besieged fronts in the east and bracing to meet an infantry assault in the south.
The escalating fighting along a more than 1000-km front line is stretching Ukraine's already outnumbered troops at a critical moment after Donald Trump won the US election, raising the prospect of possible talks with Russia.
Russia occupies a fifth of Ukraine and President Vladimir Putin has said he wants Ukraine to drop ambitions to join the NATO military alliance and retreat from four Ukrainian regions that he partially holds, something officials in Kyiv says is tantamount to capitulation.
Ukraine's armed forces commander General Oleksandr Syrskyi said he travelled to the front in Russia's Kursk region where a surprise Ukrainian incursion carved out a chunk of land in August that President Volodymyr Zelenskiy said could be used as a bargaining chip.
"(Russian forces) are trying to dislodge our troops and advance deep into the territory we control," he said on Telegram.
Some US military analysts have questioned the rationale of the Kursk operation, which extended an already long front line, creating more strain for Ukraine.
Ukraine says Russia has deployed 11,000 North Korean troops to the Kursk region and that they have already been involved in clashes, urging its allies to respond robustly.
Russia has neither denied nor confirmed their presence.
Syrskyi said the Kursk operation had drawn in crack Russian fighters and relieved pressure that would have been brought to bear on several important outposts in eastern Ukraine where Russia has been making gains at its fastest clip since 2022.
"These tens of thousands of enemies from the best Russian shock units would have stormed our positions in the Pokrovsk, Kurakhove or Toretsk directions, which would have significantly worsened the situation at the front," he said.
The Ukrainian governor of Donetsk region said a dam at the Kurakhove reservoir had been damaged creating a threat to villagers living near the Vovcha river.
He blamed Russian shelling.
Zelenskiy said that Ukraine would strengthen positions on the Pokrovsk and Kurakhove fronts where Russia has directed its offensive pressure for months.
Russia has been closing in on Pokrovsk, a strategic road and rail hub that has a coal mine.
The small industrial town of Kurakhove is home to a major coal-powered thermal power plant.
A Ukrainian military spokesperson told Reuters that Russia was also moving trained assault groups to forward positions in the southern Zaporizhzhia region and that they were preparing to attack.
The southern front has had far less fighting since 2023 when Ukraine launched a major counteroffensive that failed to break through heavily defended and mined lands held by Russia.
"(The assaults) could begin in the near future, we're not even talking about weeks, we're expecting it to happen any day," said Vladyslav Voloshyn, spokesman for the southern military sector.
Although it was not clear if they would involve a single offensive push or separate assaults, intelligence assessed that Russian troops planned to use armoured vehicles and a considerable number of drones, he said.
"They are preparing both armoured groups and light vehicles – buggies, motorcycles - to conduct these assault operations," he added.
Russia has already carried out preliminary reconnaissance and stepped up air strikes in the south by about 30-40 per cent in the last two-three weeks, using bombers and unguided air missiles, he added.
Russia has been claiming the capture of village after village as it advances in Ukraine's east, and has vowed to expel Ukrainian forces from its Kursk region.
Reuters was not able to independently verify reports from the front.
