Moscow has abandoned its primary bastion in north-eastern Ukraine as a result of a dramatic collapse of one of the war's primary frontlines caused by a quick Ukrainian assault.

In a stunning counteroffensive, Ukrainian soldiers pushed up to 50 kilometers past Russian lines this week and recovered dozens of villages.

The rapid collapse of three vital cities in Kharkiv province — Izium, Kupiansk, and Balaklyia — was Moscow's greatest loss since Russian soldiers were withdrawn from the Ukrainian capital in March.

Dmytro Kuleba, the foreign minister of Ukraine, stated that the supply of arms from the west was paying off.

"We showed that we can defeat the Russian army," he said.

"I repeat again the more weapons we receive the faster we win, the faster this war will end and we will be able to concentrate on other goals."

This could be a key turning point in the 6-month-old battle since thousands of fleeing Russian soldiers have abandoned their ammo and equipment.

Izium served as the logistical hub for one of Russia's major campaigns, a months-long attack from the north on the neighboring Donbas region consisting of Donetsk and Lugansk.

The Russian state-run news service TASS said that the Defense Ministry has instructed troops to withdraw from the area and boost operations elsewhere in neighboring Donetsk.

TASS claimed that the director of Russia's administration in Kharkiv advised residents to abandon the province and flee to Russia to "save lives"

Witnesses described traffic congestion caused by individuals fleeing territory under Russian control.

Advances cut Russian supply lines

Ukrainian politicians proclaimed Russia's retreat.

President Volodymyr Zelenskyy stated in a video address on Saturday that Ukraine's military forces have reclaimed around 2,000 square kilometers of territory since the beginning of this month's counteroffensive against Russia.

Andriy Yermak, Mr. Zelenskyy's chief of staff, uploaded a photo of forces on the outskirts of Izium along with a grapes emoji on Twitter. Ukrainian officials refrained from declaring that Izium had been regained. The name of the city means "raisin"

The Russian army claimed to be the "fastest army in the world … keep running!" Later, Yermak wrote on Twitter.

The news of Russia's departure came hours after Ukrainian forces conquered the northern city of Kupiansk, the only railway hub supporting Russia's entire front line in north-eastern Ukraine.

Saturday morning, Ukrainian officials uploaded photographs of their troops hoisting the blue-and-yellow flag in front of Kupiansk's municipal hall.

Along a front that has witnessed some of the fiercest fights of the conflict, hundreds of Russian troops were abruptly cut off from their supplies.

There were indications of trouble elsewhere along Russia's remaining eastern front posts, as pro-Russian officials acknowledged challenges in other regions.

Mr. Zelenskyy stated that the Ukrainian armed forces continue to advance in several places along the front.

'Russia is retreating'

A witness in Valuyki, a town in the Belgorod area of Russia near the Ukrainian border, reported seeing Kupiansk families eating and sleeping in their cars along roadways.

"Today, I was at the market and saw several folks from Kupiansk. According to reports, the Ukrainian army has captured half of the city, and Russia is retreating... The fighting is intensifying "the witness stated.

Governor of Belgorod Vyacheslav Gladkov stated that officials were providing food and medical aid to those waiting at a Russian border crossing.

Senator Andrey Turchak of the pro-Kremlin party United Russia said that more than 400 vehicles were present at the border.

According to Ukrainian sources, Russian rocket fire struck Kharkiv city on Saturday evening, killing at least one person and causing damage to many residences as part of an increase in bombardment since Kyiv's counteroffensive.

Neither account of the conflict could be independently corroborated.

On the eastern front, there were indications that Ukraine could capitalize on the chaos by launching attacks in other regions.

Denis Pushilin, the president of the separatist administration placed by Russia in Donetsk province, stated that the situation in Liman, east of Izium, "remains quite difficult — as in a series of settlements in the north of the republic"

A bit further east, Ukrainian officials hinted at a potential push to reclaim Lysychansk, which Moscow captured in July after weeks of combat in one of the bloodiest engagements of the conflict.

The media reported that the regional governor of Ukraine, Serhiy Gaidai, had detected Ukrainian troops within the city's boundaries.

The city's name means "fox" and Mr. Zelenskyy's chief of staff, Andriy Yermak, tweeted a fox emoji in response to his message of grapes.