Some 31,000 Ukrainian soldiers have been killed since Russia’s full-scale invasion began two years ago, President Volodymyr Zelensky said on Sunday, acknowledging for the first time in the war a concrete figure for Ukraine’s toll.

“This is a big loss for us,” Mr. Zelensky said at a news conference in Kyiv, the Ukrainian capital. But he declined to disclose the number of wounded or missing, saying that Russia could use the information to gauge the number of Ukraine’s active forces.

Mr. Zelensky’s tally could not be independently verified. It differs sharply from estimates by U.S. officials, who, this past summer, put the losses much higher, saying that close to 70,000 Ukrainians had been killed and 100,000 to 120,000 had been wounded. Russia’s military casualties, the officials said, were about twice as high.

By revealing Ukraine’s losses, Mr. Zelensky said he wanted to counter Russian propaganda and other estimates that have placed Ukrainian casualties at a much higher level.

Mr. Zelensky’s unusual acknowledgment came as his country’s armed forces are now on the back foot along most of the 600-mile front line, with Russian troops pressing attacks in the east and south. A week ago, Moscow captured the city of Avdiivka, a Ukrainian stronghold in the east, and its troops have been slowly pushing westward in recent days, trying to build on their momentum in the area.

Ukraine’s top general, Oleksandr Syrsky, said he had ordered his troops to withdraw from Avdiivka to “preserve the lives and health of the soldiers,” which he described as the army’s “highest value.”

While Moscow has sent wave after wave of troops to capture cities and towns, regardless of the losses, Ukraine has a much smaller pool of soldiers to draw from and needs to preserve its forces.

Ukraine’s military leaders have long said that they need more troops as the war drags on to make up for losses on the battlefield and withstand another year of fierce fighting. A mobilization bill that could pave the way for a large-scale draft of up to 500,000 soldiers is currently making its way through Ukraine’s Parliament. But many in Ukraine fear that a mass mobilization could stir up social tensions.

Trying to project optimism, Ukraine’s government assembled ministers and top officials at a conference in Kyiv on Sunday to unveil plans for the future, as the country enters its third year of full-scale war with Russia.