Ukraine recaptures swath of land in 'breakthrough' offensive

Ukrainian forces have made a major breakthrough in the Kharkiv region, capturing strategically important cities, according to Ukrainian officials and military sources.

Ukrainian troops have advanced 70 kilometers in less than a week, resulting in part of Russia's front-line collapse in Ukraine's northeast, with large numbers of Russian troops forced to retreat.

One Ukrainian military source described the multi-pronged offensive as "a thoroughly planned operation" and told ABC News that "everything was running to schedule."

The source also claimed that Russian troops in the area "were running away."

In a major development on Saturday, Ukrainian forces appeared to have retaken the strategically important city of Izyum.

This claim was made by multiple unconfirmed reports on social media and confirmed by a military source to ABC News. Russian military bloggers, some of who are embedded with Russian troops in Ukraine, also said Russian forces had retreated from Izyum to avoid being encircled.

Izyum was Russia's headquarters for its offensive into the neighboring Donetsk region.

Russia's Defense Ministry on Saturday confirmed it has carried out what it called a "regrouping" of its forces around Izyum. Russian military bloggers wrote on social media in reality Russian forces had retreated, while Russian state media has described the situation as "extremely difficult."

The dramatic advance by Ukrainian forces also reaffirms how much the war has shifted in Ukraine's favor and adds credence to claims by U.S. and British officials that Russian forces are overstretched.

The retaking of the city also shows the impact that weaponry from the U.S. and its allies is having on the battlefield.

"U.S. assistance is arriving where it is needed," former U.S. ambassador to NATO, Kurt Volker told ABC News. "The Ukrainians know how to use it."

"The Russians are militarily exhausted," Volker claimed, and called on U.S. President Joe Biden's administration to provide even longer-range rockets to Ukraine.

Ukraine’s counteroffensive in the northeast began late last week and swiftly broke through Russia’s line there. Since then Ukrainian forces had recaptured the city of Balakliya and on Saturday seized Kupiansk, a key logistics hub, according to Ukrainian officials as well as Russian pro-government journalists.

The rapid offensive in the northeast has seen Ukrainian forces recapture the cities of Balakliya and Kupiansk, according to Ukrainian officials.

Photos and videos of troops raising the Ukrainian flag are circulating online.

Kupiansk is well-connected by rail and road -- making it a vital link in Russian supply lines in the area. By seizing the two cities, Ukrainian forces threatened to encircle completely thousands of Russian troops in Izyum, making its defense untenable and forcing Russia to withdraw them.

On Friday night, Ukrainian President Volodymyr Zelenskyy said his forces had liberated and gained control of 30 settlements in the northeast.

The gains amounted to 2,500 square kilometers, according to the Institute for the Study of War.

Ukrainian forces also claim to have retaken several villages in the southern region of the country in recent days.

Unlike the offensive in the northeast, the southern offensive was widely publicized by Ukrainian officials, raising the possibility that the Russian military was distracted by events in the south, and caught off-guard when the Ukrainian military went on the offensive in the northeast.