KHARKIV, Sept 12 - Ukrainian troops expanded their territorial gains Monday, pushing all the way to the country’s northeastern border in places, and claimed to have captured a record number of Russian soldiers as part of the lightning advance that forced Moscow to make a hasty retreat.

A spokesman for Ukrainian military intelligence said Russian troops were surrendering en masse as “they understand the hopelessness of their situation,” The Associated Press reported. A Ukrainian presidential adviser said there were so many POWs that the country was running out of space to accommodate them.

Blue-and-yellow Ukrainian flags fluttered over newly liberated towns across a wide swath of reclaimed land. 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.

“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.

The counteroffensive left the Kremlin struggling for a response to its largest military defeat in Ukraine since Russian forces pulled back from areas near Kyiv after a botched attempt to capture the capital early in the invasion.

The Russian Defense Ministry acknowledged the setback in a map that showed its troops pressed back along a narrow patch of land on the border with Russia — a tacit admission of big Ukrainian gains.

In his evening address, President Volodymyr Zelenskyy said his forces have liberated more than 6,000 square kilometers (2,300 square miles) in the east and the south since the beginning of September.

Now Ukrainian teams are disarming land mines and other unexploded weapons in the recaptured areas and searching for any remaining Russian troops, officials said.

Ukrainian presidential adviser Oleksiy Arestovich did not specify the number of Russian prisoners but said the POWs would be exchanged for Ukrainian service members held by Moscow. Military intelligence spokesman Andrey Yusov said the captured troops included “significant” numbers of Russian officers.

Ukraine’s deputy interior minister accused fleeing Russian forces of burning official documents and concealing bodies in an attempt to cover up rights violations in the areas they controlled until last week.

Pro-Kremlin separatists reported that Ukrainian troops were approaching the town of Lyman, a rail hub captured by Russia in late May that offers access to bridges over the nearby Siversky Donets river.

Ukraine’s presidential office said Monday that at least four civilians were killed and 11 others wounded in a series of Russian attacks in nine regions of the country. The U.N. Human Rights Office said last week that 5,767 civilians have been killed so far. (ap/ez)