Russian forces have scored small territorial gains along Ukraine’s eastern front in recent weeks, using their manpower advantage in grueling battles and prompting the Ukrainian authorities to consider a push to mobilize up to 500,000 soldiers to sustain the exhausting fight next year.

The chief of Ukraine’s military intelligence, Kyrylo Budanov, said this week that he saw no alternative to a large-scale mobilization to make up for Ukraine’s losses. And President Volodymyr Zelensky said his army chiefs had asked him to mobilize 450,000 to 500,000 men.

“This is a serious number,” Mr. Zelensky said at a news conference on Tuesday, adding that a plan still had to be drawn up before he could make a decision. Although many Ukrainians have yet to be drafted into the military, the authorities are reluctant to resort to mass conscription for fear of stirring up social tensions.

Most recently, Russian troops have been closing in on the city of Avdiivka, a Ukrainian stronghold in the Donetsk region. The capture of Avdiivka would be a strategic success for Russia — the city is a linchpin of Ukrainian defenses in the region — and would deal a blow to Ukrainians’ morale. The city is a symbol of resistance, having withstood nearly a decade of warfare since Russian-backed forces first tried to seize it in 2014.