Ukrainian forces kept pushing north in the Kharkiv region and advancing to its south and east, Ukraine’s army chief said on Sunday, a day after their rapid gains made Russia abandon its main bastion in the area.

President Volodymyr Zelensky hailed Ukraine’s advance in the northeast Kharkiv province as a potential breakthrough in the six-month-old war, saying this winter could bring more rapid gains of territory if Kyiv can get more powerful weapons.

“In the Kharkiv direction, we began to advance not only to the south and east, but also to the north. There are 50km to go to the state border (with Russia),” Ukraine’s chief commander General Valeriy Zaluzhnyi said on Telegram.

He said the country’s armed forces have regained control of more than 3,000 square km since the beginning of this month.

In Moscow, Russia’s defence ministry said on Sunday that Russian forces were hitting Ukrainian army positions in the Kharkiv region with precision strikes delivered by airborne troops, missiles and artillery.

The retreat from the city of Izium marked Russian forces’ worst defeat since they were pushed back from the capital Kyiv in March, as thousands of Russian soldiers left behind ammunition and equipment as they fled.

The gains are important politically for Zelensky as he seeks to keep Europe united behind Ukraine — supplying weapons and money — even as an energy crisis looms this winter following cuts in Russian gas supplies to European customers.

“I believe that this winter is a turning point, and it can lead to the rapid de-occupation of Ukraine,” Zelensky said in comments to a political forum published on his website late on Saturday. “We see how they (occupiers) are fleeing in some directions. If we were a little stronger with weapons, we would de-occupy faster.”

Ukrainian officials stopped short of confirming they had recaptured Izium, but Zelensky’s chief of staff Andriy Yermak posted a photo of troops on its outskirts and tweeted an emoji of grapes. The city’s name means “raisin”.

Kyiv-based military analyst Oleh Zhdanov said the gains could pave the way for a further push into Luhansk region, whose capture Russia claimed at the beginning of July.

“If you look at the map, it is logical to assume that the offensive will develop in the direction of Svatove — Starobelsk, and Sievierodonetsk — Lysychansk. These are two promising directions,” he said.

Anger in Russia

Russian nationalists called angrily on Sunday for President Vladimir Putin to make immediate changes to ensure ultimate victory in the Ukraine war.

As Russian forces abandoned town after town on Saturday, Putin was opening Europe’s largest ferris wheel in a Moscow park, while fireworks lit up the sky over Red Square to celebrate the city’s founding in 1147.

Rumblings of disquiet even spread to Russia’s ordinarily subservient parliament.

Sergei Mironov, leader of the nominally opposition but Putin-loyal Just Russia party, said on Twitter that a firework display in honour of Moscow Day should be cancelled, in view of the military situation.

One message reposted on Telegram by the prominent war correspondent Semyon Pegov referred to the celebrations in Moscow as “blasphemous”.