Ukrainian President Volodymyr Zelenskyy on Monday said Ukrainian forces had recaptured 6,000 square kilometers (2,320 square miles) of territory from Russia in a counter-offensive this month.
"Since the start of September, our soldiers have already liberated 6,000 square kilometers of Ukrainian territory in the east and south, and we are moving further," Zelensky said in his daily address, as quoted by the AFP news agency.
Ukraine has claimed sweeping successes in its lightning counter-offensive in Kharkiv region in the northeast, which saw dozens of areas, including the cities of Izyum, Kupiansk and Balakliya, retaken.
It has also claimed significant gains in the southern Kherson region, where the Ukrainian army said earlier on Monday it had recaptured 500 square kilometers.
A senior US military official, meanwhile, said that Russia has largely ceded its gains near Kharkiv and many of the withdrawing Russian soldiers have exited Ukraine, moving over the border back into Russia.
As it pulls back, the United States has seen anecdotal reports of Russian forces abandoning their equipment, "which could be indicative of Russia's disorganized command and control," the US military official said, according to the Reuters news agency.
The US military official was upbeat, but cautious, when describing the Ukrainian advances.
"It's clear they're fighting hard," the official said, citing progress in the south and east to reclaim territory.
Kharkiv region had been partly occupied by Russian troops since the start of the invasion launched on February 24.
The city of the same name is regularly targeted by bombardments, but the Russian troops have never managed to seize it.