President Volodymyr Zelenskyy said on Monday that Ukrainian forces have retaken 6,000 square kilometers of territory captured by Russia since the beginning of the month, Reuters reports.

“From the beginning of September until today, our fighters have liberated more than 6,000 square kilometers of the territory of Ukraine in the south and east,” Zelenskyy said in a daily video message. “The advance of our forces continues.”

The Commander-in-Chief of Ukraine, General Valery Zaluzhnyi, said on Sunday that his forces had recaptured more than 3,000 square kilometers this month.

The United States believes Russia has largely ceded its captured territories near Kharkiv, and many retreating Russian soldiers have left Ukraine, crossing the border back into Russia, a senior US military official said on Monday.

“In general, we assess that the Ukrainians are making progress in the struggle to liberate and restore territory in the south and east,” an American military official told reporters on condition of anonymity.

“On the spot, near Kharkiv, we estimate that the Russian forces have mostly surrendered their gains to the Ukrainians and retreated to the north and east. Many of these forces crossed the border into Russia.”