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.