KYIV, Ukraine — Russian forces on Tuesday inched closer to the central part of Vovchansk, a town in Ukraine’s northeast that they have been attacking for the past 10 days as part of a new offensive in the region.

Roman Semenukha, the deputy head of the military administration in the northeastern Kharkiv region, said on television Monday that Ukrainian forces had lost about 40% of the town, with Russian troops pushing from the north.

Open-source maps of the battlefield compiled by independent groups also show that Russia now controls the northern part of the town, which had a prewar population of 17,000.

Vovchansk, which lies just 5 miles from the Russian border, has been a prime target of Moscow’s new offensive. Both U.S. officials and President Vladimir Putin of Russia have said the offensive is part of an effort to establish a buffer zone.

The intention, analysts and military officials say, is to push Ukrainian forces away from the border, to prevent them from targeting Russian towns and cities with artillery.

Russian forces have also advanced to the west of Vovchansk, pushing about 5 miles into Ukrainian territory just north of Kharkiv, Ukraine’s second-largest city. They now control more than 70 square miles of Ukrainian land in the area and have made their largest territorial gains since the end of 2022.

Ukrainian officials have said in recent days that the situation in the region has stabilized thanks to the urgent redeployment of Ukrainian units that have managed to slow the Russian advance.

But President Volodymyr Zelenskyy of Ukraine warned in an interview with news agency Agence France-Presse that the assaults may be only the “first wave” in a wider offensive designed to stretch Ukraine’s already outnumbered and outgunned forces.

For now, Russian troops appear determined to capture Vovchansk. They have been pounding the town for days with powerful guided weapons known as glide bombs that can deliver hundreds of kilograms of explosives on impact, Ukrainian officials said.

After reaching the outskirts of the town in the first days of their offensive 10 days ago, Russian forces slowly pushed inside it, engaging in street fighting.

Ukrainian authorities have been scrambling to evacuate residents from the town who are under fire.