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 on Monday that Ukrainian forces had lost about 40 percent 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 five miles from the Russian border, has been a prime target of Moscow’s new offensive Both U.S. officials and President Vladimir V. 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.

“The Kremlin’s forces will try to make further advances in the weeks ahead, and try to carve out a buffer zone along the Ukrainian border,” Lloyd J. Austin III, the U.S. defense secretary, said on Monday during a meeting with Western allies of Ukraine.