Russian forces have seized at least nine villages amid a new push in northeastern Ukraine that began on Friday, the largest advance by Russian forces since the start of the full-scale invasion in February 2022.

On Friday, Russian troops pushed deeper into territory in two border areas of Vovchansk and Lyptsi in Kharkiv region. On Saturday, they took five villages and on Sunday another four.

Meanwhile, Nazar Voloshyn, a spokesperson for the Ukrainian military, told RBC-Ukraine that Yuri Galushkin, the commander responsible for the northeastern front, has been replaced by . Russia too, just experienced a major reshuffling, with President Vladimir Putin announcing on Sunday that he had replaced Sergei Shoigu with economist Andrey Belousov.

“This week, the situation in Kharkiv Oblast has significantly worsened. Currently, there are ongoing battles in the border areas along the state border with the Russian Federation,” Oleksandr Syrskii, Ukraine’s army chief, wrote . “It is a difficult situation, but the Defense Forces of Ukraine are doing everything to hold defensive lines and positions and inflict damage on the enemy.”

Ukrainian President acknowledged the situation in a video message on Sunday. “Defensive operations and fierce battles are taking place in the Kharkiv region along a significant border strip,” he wrote. “Some villages have effectively turned from a grey zone to a war zone. Occupiers are attempting to seize control of some of them while using others to advance.”

Ukraine has struggled in recent months. A combined with a dwindling supply of soldiers is forcing Ukrainian forces to retreat as Russians attack from both air and land. “It’s necessary to increase the pace of building fortifications … so that when we retreat, we will retreat to a prepared position,” a unit commander , a week before Russia’s latest push in Kharkiv. “These fortifications are not enough.”

The U.S. announced another on Friday that will include , munitions for Patriot and National Advanced Surface-to-Air Missile Systems, artillery, anti-aircraft and anti-tank munitions, and armored vehicles.

But it will take months for many of these supplies to reach the frontline. Russia is taking advantage of this time gap by launching this offensive push, says Oleksandr V. Danylyuk, an expert in Russian multidimensional warfare at the London-based RUSI think-tank. “They believe that they have a window of opportunity during these next few months, and they will do the best to achieve more territorial gains,” he tells TIME. “It’s clear that Ukraine is in a critical shortage of ammunition specifically and the most important part of military capabilities in this war is artillery.”

Russia’s latest push follows months of dug-in warfare where the frontlines have barely moved. It also comes as border regions such as Belgorod are routinely shelled by Ukraine.

In March, Russia announced plans to evacuate 9,000 children from the city to parts of Russia less impacted by the war. On Sunday, when fragments of a Ukrainian missile intercepted by Russia hit an apartment building in the city.