Ukrainian forces said they accepted the surrender of the largest single group of Russian soldiers since the start of the war more than two years ago, as Kyiv’s military claimed to continue expanding its cross-border incursion.

A Ukrainian Security Service unit operating in Russia’s Kursk region took 102 Russian servicemen as prisoners-of-war, according to a person with knowledge of the operation, who asked not to be identified because the matter is sensitive.

The Russians were captured Wednesday in a sprawling underground complex, and had ample stocks of ammunition and supplies, the person said. Russia hasn’t commented.

Ukraine now controls 444 square miles of Russian territory, including 82 villages and towns, since the incursion that began 10 days ago, Commander-in-Chief Oleksandr Syrskyi told President Volodymyr Zelenskyy in a video report posted on Zelenskyy’s Telegram channel today. The claims couldn’t be independently verified.

Pentagon spokeswoman Sabrina Singh told reporters today that the U.S. was “still trying to learn more” about Ukraine’s objectives in its offensive in Russia after U.S. Defense Secretary Lloyd Austin spoke by phone to Ukrainian Defense Minister Rustem Umerov on Wednesday.

Early reports show some Russian units have been redirected from Ukraine to the Kursk area to help stymie the effort, Singh said, adding she didn’t yet have an assessment on whether Russia’s supply lines to occupied territories in Ukraine, including Donbas, have been affected.

The Ukrainian move into Russia is the latest challenge to efforts by the U.S. and other nations supporting Ukraine to restrict the use of their weapons in Russia except for limited self-defense purposes.

“The fact that Ukraine has now been able to use not just U.S., but an array of Western-supplied equipment, in this operation shows that anyone who thought that there was a hard red line at Ukraine striking into Russia had talked themselves into that red line — and in reality, it didn’t exist,” said Chris Miller, a nonresident senior fellow at the American Enterprise Institute.

Zelenskyy said Ukraine has set up a military command post in the Kursk regional town of Sudzha, which Syrskyi said would be used to maintain public order and provide for the basic needs of the local population. The president praised Ukraine’s troops in the region for replenishing the “exchange fund” for prisoner swaps in his regular video address on Wednesday.

Russian officials have already contacted Ukraine about a possible prisoner exchange, Dmytro Lubinets, the parliamentary human rights commissioner in Kyiv, said in televised comments Wednesday. He declined to give details, including on the total number of servicemen who’ve been captured.

Russian human rights ombudsman Tatiana Moskalkova, who represents the Kremlin in negotiations on prisoner swaps, didn’t immediately respond to a request to comment.

Russian Defense Minister Andrey Belousov held talks with military officials “on ensuring the security of the population” in border regions, the Defense Ministry in Moscow said in a post today on Telegram.

Belousov discussed measures to ensure “the integrity and inviolability” of the Belgorod region, which neighbors Kursk, against Ukrainian attacks, saying he’d personally monitor their implementation, according to the statement.

The authorities in Moscow declared a federal state of emergency in Belgorod today. Regional Governor Vyacheslav Gladkov said it was facing repeated drone attacks and shelling from Ukrainian forces.

Fighting continues in eastern Ukraine where Russian forces are advancing to within 10 kilometers (6 miles) of the Donetsk region city of Pokrovsk. The head of the local administration, Serhii Dobriak, urged residents to leave, saying in a video on its website that “the situation is only getting worse” in the city that’s an important logistics hub in the region for Ukraine.

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With assistance from Courtney McBride and Natalia Drozdiak.