KHARKIV, Ukraine -- Ukrainian troops reclaimed a wide swath of territory from Russia on Monday, pushing all the way back to the northeastern border in some places and capturing many Russian soldiers as part of a lightning advance that forced Moscow to make a hasty retreat.

A spokesman for Ukrainian military intelligence claimed that Russian troops were surrendering en masse as “they understand the hopelessness of their situation.” A Ukrainian presidential adviser said the country was running out of space to accommodate the POWs.

As blue-and-yellow Ukrainian flags fluttered over newly liberated towns, the Ukrainian military said it had freed more than 20 settlements in 24 hours. In recent days, Kyiv’s forces have captured territory at least twice the size of greater London, according to the British Defense Ministry.

After months of little discernible movement on the battlefield, the momentum has lifted Ukrainian morale and provoked rare public criticism of Russian President Vladimir Putin’s war. While Ukrainian cities began emerging from Russian occupation, a local leader alleged that the Kremlin's troops had committed atrocities against civilians there similar to those in other places seized by Moscow.

“In some areas of the front, our defenders reached the state border with the Russian Federation,” said Oleh Syniehubov, governor of the northeastern Kharkiv region. Over the weekend, the Russian Defense Ministry said troops would be pulled from two areas in that region to regroup in the eastern region of Donetsk