(CNN) — When Ukrainian forces entered the city of Izium Saturday, it was more than a major military victory. It was a sign the war in Ukraine might be entering a new phase, one in which Russian troops are scrambling to hold onto the territory they captured over the past six months.

Russian forces were forced to flee the strategic eastern city just five days after Ukrainian forces began a new offensive eastward through the Kharkiv region.

“Russians escaped and left weapons and ammo behind. City center is free,” a spokesperson for the Bohun Brigade of the Land Forces of Ukraine said in a statement Saturday afternoon.

The last five days have seen the most ambitious ground assaults by the Ukrainians since Russia launched its full-scale invasion in late February. Video and satellite images geolocated by CNN show the advances have involved sustained attacks on command posts, ammunition stores, and fuel reserves far behind the front lines.

A senior US official said Ukrainian forces had achieved some success in attacking Russian supply lines, with the intention of cutting off and isolating Russian troops west of the Dnipro River.

Saturday’s rapid advance did not end with Izium, as Ukraine appeared to have opened a new front against Russian defenses on the border of the Donetsk and Luhansk regions.

The head of the regional military administration for Luhansk, Serhiy Hayday, indicated the city of Lysychansk was the target of the new offensive.

Lysychansk was the last city in the Luhansk region in eastern Ukraine to fall under Russian control in July, after weeks of intense fighting. Hayday told CNN on Saturday “the occupiers, including both the collaborators and the military, are running away in a hurry.”

“Locals have videos and photos proving that,” Hayday said. He said the visual evidence could not be shared for security reasons. CNN was unable to verify Hayday’s claim, but at least one border crossing into Russia has seen lines of vehicles forming.

Oleksiy Reznikov, the Ukrainian defense minister, said Saturday Ukraine’s allies are “amazed” by the recent successes of the country’s military. Speaking at the end of a meeting of the Ukraine Defense Contact Group in Ramstein Air Base, Germany, Reznikov said Ukraine’s “partners were positively amazed by the dynamics of our troops’ activities” in the south and east of the country.

Russian reinforcements

As Russian forces scramble to counter the Ukrainian advance in the east, Moscow is trying to reinforce its embattled military units there.

Video footage taken by a Russian military journalist Yevgeniy Poddubny Friday showed Russian helicopters arriving in the region and at least one unloading an armored vehicle.

Poddubny reported Friday Russian military headquarters was using Mi-26 helicopters to reinforce units in Kharkiv with both men and armored vehicles, redeploying reserve troops to both Kupyansk and Izium.

But the reinforcements did not seem to have helped. In a big strategic win for Kyiv, both Izium and Kupyansk were recaptured.

Izium, which sits near the border between the Kharkiv and Donetsk regions, had been under Russian occupation for more than five months and had become an important hub for the invading military. Russia was using Izium as a launching pad for attacks southward into the Donetsk region and Kupyansk, some 30 miles to the north of Izium, as a rail hub to resupply its forces.

Several videos of Ukrainian troops standing at the Izium sign at the entrance to the city were posted on Telegram. One piece of footage was purportedly filmed by a reconnaissance unit of Ukraine’s 25th Paratroopers’ Brigade. Over the video, a voice is heard saying: “We are in Izium. Everything will be Ukraine. Our flag is here already.”

Kyiv said its forces have also severed a major Russian supply line through the Kharkiv region and were moving further east. At the same time, the Ukrainian military said losses have been inflicted on Russian forces in the Kherson region of southern Ukraine.

Even the Russian-backed authorities in Izium were forced to admit the retreat on Saturday. The head of the pro-Russian Izium city administration Vladislav Sokolov was quoted by the Russian state news agency RIA as saying the situation in Izium was “very difficult.”

Sokolov was later quoted by the Russian state news agency TASS as saying: “We have begun the evacuation, albeit at a slow pace, but as far as possible, we are taking them to the territory of the Russian Federation.”

Kirill Imashev, the military correspondent of a Russian Telegram channel, Readovka, said Russian forces had left Kupyansk and retreated across the river Oskil to “regroup,” but claimed residents of the city had “left their homes in a hurry, fearing reprisals from the Kyiv regime.”

“Hundreds of cars are leaving from Kupyansk to the LPR [the self-declared Luhansk People’s Republic],” he said.

The available evidence on social media networks shows the civilian population in the areas liberated have greeted Ukrainian forces with enthusiasm. Ukrainian officials have vowed anyone who collaborated with occupation forces will face criminal sanctions.

Ukraine said some Russian troops are now deserting due to the “significant losses” they have suffered. The General Staff of the Ukrainian Armed Forces said Friday at least 15 desertions had taken place. It said soldiers were abandoning their uniforms and wearing civilian clothes in an effort to blend in while attempting to return to Russian territory, it said.

Meanwhile, Moscow was trying to paint a different picture of the Ukrainian advance.

In its first response to the gains made by Ukrainian forces in the last few days, the Russian Defense Ministry has said “the decision was made to regroup Russian troops in the areas of Balakleya and Izyum and redirect their efforts in the Donetsk direction.”

The advancing Ukrainian troops have reported scenes of destruction as they retake areas from Russian control.

The head of the Kharkiv region military administration, Oleh Syniehubov, said he had toured recently-liberated areas where the Russians had “left broken infrastructure and houses, many trophies and piles of garbage,” and posted photographs of the scenes on Telegram.

“People, without exaggeration, met our soldiers with tears in their eyes,” Syniehubov said. “Residents, of course, need help. Most villages do not have electricity or gas. In the near future, we will restore them and bring in humanitarian aid.”

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