Over the course of three days, Ukraine tripled the amount of land it took back from Russia.
Since the beginning of September, Zelenskyy said, Ukraine had regained 386 square miles of land from Russia.
He said on Saturday that this had gone up to over 800 square miles.
On Sunday, General Valeriy Zaluzhnyi, the commander in chief of the Ukrainian armed forces, said that the Ukrainians had taken back 1,158 square miles of Russian-occupied territory.
Insider is unable to verify the figures, and Ukraine has not allowed the media to report on the counteroffensive.
Russia has confirmed that its troops have left the area.
The UK Ministry of Defence said on Sunday that Russia had likely withdrawn units from the area and that Ukraine had made significant gains in the region.
According to the Institute for the Study of War, the territory taken by the Ukrainians so far this month was more territory than the Russians have captured.
The main goal of the Ukrainian army is to take back all of the land from Russia.
Mariupol city council wrote on the official Telegram channel that the Ukrainian forces might be preparing for a quick breakthrough to Mariupol.
According to officials on Telegram, the Ukrainian government has regained control of the town three kilometers from the Russia border.